I'll try to address this in two parts. (Otherwise with my tendency to ramble it'll go on forever!) So here's Part I, getting from a POD through the 1980 elections that manage to produce a Democrat in the White House.
I think for the kind of thing the OP is describing (I will get to
@Dunning Kruger's point which is essentially about how the lines were drawn for partisan realignment, and as much as realignment a partisan
orientation that like most things "Sixties" started out then but didn't really gather steam until the Seventies) the POD is Ford being reelected and, for what the OP has asked, I think it's an
essential POD. You need that or you need a
markedly different Democrat in 1976 and I don't mean another outside-the-box technocratic prophet like Jerry Brown (a West-Coast-culture equivalent for Carter in a number of ways), I mean someone like a Hubert Humphrey who stayed just healthy enough to get through all or most of one term and left behind them a successor to carry on. Otherwise I think you need to start with Ford for a number of reasons.
First, there is the
way Ford nearly came back to win in 1976. Change less than thirty thousand votes in Ohio and Mississippi (the later, say, through Reagan campaigning harder for him down there, the former because butterflies are pretty) and you have Ford as president. But
how has he gotten there? He's lost the popular vote by a margin of nearly two percent, like Trump-v-Clinton bad but with less third-party meddling. But (almost more like 2000 than 2016) there
is third-party "meddling" because Carter lost a series of smaller states from Maine to Iowa to Oregon by less than the number of angry lefty Democrats who protest-voted for Gene McCarthy. So the president with an asterisk by his name now has an even bigger asterisk by it (first unelected president, now first in the 20th century to lose the popular vote and win the EC.) That doesn't help. Ford has to deal with all the same issues in the later Seventies. He hopefully manages to not be drawn into any small scale splendid little wars by Henry Kissinger feeling his manhood is fading (and Henry will go during this term from some combo of sheer exhaustion, losing bureaucratic battles to Rumsfeld and Cheney, and being pushed out to placate the New Right. Probably he gets replaced by Poppy Bush.) His vigorously right-wing Treasury Secretary, William Simon, is going to push for more Wall Street deregulation several years ahead of OTL's Don Regan (they were buddies, as it happens), and in keeping with the strongly fiscally conservative Ford will fight hard to limit the scale of any stimulus packages the Democratic Congress is pushing. There will be some increases in spending in terms of defense, but only some (Rumsfeld will probably get bogged down fighting for the B-1 boondoggle and at some point, now that James Baker -- that's Bush
consigliere Baker -- has
successfully managed Ford's campaign, when Cheney's ego leads him towards politics they will combine resources to marginalize Rumsfeld, probably trying to push him out of the Pentagon where Rummy was deeply unpopular even then.) So the recovery of 1977-78 will probably be a bit weaker, but also there will be more efforts at clamping down on inflation and Ford will talk these up. If he hasn't goofed other things too badly he will be in semi-decent shape around '78 or so just as Carter was. He will do some things that are deeply unpopular with the New Right, in particular pushing a Panama Canal Treaty (to prevent another war in another jungle) and pressing for the Equal Rights Amendment. So he'll need to throw them a bone or two and one at least will be trying to get some right-wing jurist (maybe Bork, maybe a young Scalia or J. Harvie Willkinson) onto the Court when Potter Stewart retires because there's a friendly POTUS to nominate his replacement. That, plus pushing against labor law reform, plus a relatively jobs-weak recovery, will all give the Democratic Congress ammunition.
Where it all goes to hell is foreign policy, and that starts really by around the beginning of '79? Which quasi-fascist government does the US tacitly back if there's a Beagle Islands war between Chile and Argentina and how bad are the optics? How involved does the US get trying early on to suppress revolt in El Salvador and in trying to prevent the Sandanistas from taking advantage of the broad popular movement to topple Somoza? How much does that raise hackles so shortly after Vietnam? Also, the US will get sucked further and further into propping up Iran as the Shah is dying (as I pointed out in another thread, probably aiming for some junta-as-trusteeship for the Shah's teenage son) and the Soviets will get sucked further into Afghanistan. Why? Because for reasons that are not easily butterflied Washington and Moscow's approaches, respectively, to those two countries were fundamentally
reactive rather than proactive for most of the Seventies. So you get the Soviets moving into Afghanistan with a more conservative government in DC (albeit one committed to détente) reacting, and you get a much more aggressive effort to prop up the Peacock Throne's house of cards by gunning people down in the streets. But that's really only going to last as long as the Shah, tops: by the time he's unfit to function much less dead, with a zit-faced boy on the throne and a coterie of generals as incompetent as they were corrupt in place, as soon as the average Iranian corporal is tired of making more martyrs in the streets it's all going to go to hell. With it there will be attacks on Americans fleeing the country, there will be serious American saber-rattling because that's what the Ford administration did instinctively any time it looked weak, there will be an oil shock hitting even closer to the election so the bow wave of American anger that cratered Carter's popularity between January and summer of '79 will happen even closer to the election. There's a decent probability that there will be some kind of hostage crisis, too, and while Ford was a prudent enough man (
Mayaguez aside) to not just charge in or set the Iranian oil patch ablaze with B-52s, things will be getting scary in the Middle East, the hard right will get after Ford for "losing" Iran because it's primary season and all Bob Dole's challengers except Howard Baker and maybe (if he runs ITTL) John Anderson will be from the right, inflation will shoot up again, and the Ford administration's aggressively orthodox efforts to corral inflation will help unemployment shoot up too. So there's all that going on.
Then there's the primaries. I still suspect, so long as John Sears either behaves better or gets fired as IOTL, those belong to Reagan. He voices
@Dunning Kruger's partisan dividing line better than anyone. He knew how to win crossover Wallace voters by being racist without being bigoted (i.e. attacking those to his left for favoring minorities over whites, rather than because he hated minorities -- though "strapping young bucks" was pushing it), he had all the charisma Bob Dole didn't, and he'd nearly beaten a sitting president four years prior. Dole's campaign will have ironclad institutional ties and will fight hard, and it will not be easy for Reagan. But Reagan can run criticizing Dole for Ford's problems both in general terms
and from the right. This will, however, be a different campaign year from OTL's 1980. To win the nomination, Reagan is going to have to
stay to the right longer in the primaries because TTL's Bob Dole has the institutional and fundraising powers of the heir apparent and has to defend the incumbent's record. Reagan can't just say "I'm a sunny bright tomorrow not this gloomy Carter you all hate" and pretend to moderate. He will say and do things he needs to get enough delegates that can be hung round his neck in the general, just as it would've been the case if he'd taken the nomination in '76. And if there
is any John Anderson-like movement it's more likely to hurt Reagan by siphoning liberal Republicans (already relabeling as "independents" in some places) because the party in opposition is usually more unified (we're seeing that now among Democrats even as they have little to no parliamentary leverage with their Congressional minorities.)
So what
about the Dems? Who runs, for one? Probably Carter does not, despite being the popular vote winner last time he managed ITTL to blow a tremendous summer lead and is seen as snatching defeat from the jaws of victory. John Glenn's a little too new and up for Senate reelection. Birch Bayh is up for reelection to and dealing either with illness or death of his beloved wife Marvella (her breast cancer had been aggressive and repetitive since 1972, ITTL she died in '79.) I think the likeliest folks are:
- Hugh Carey: the man who saved New York, a guy who's proven he can overcome economic bad times, who had (quietly) a record as one of the first anti-Vietnam congressmen (Carey was also a decorated WWII vet) and sponsored some of the most important legislation benefiting children in the Sixties, a good Catholic who can help keep "Catholic ethnics" and unionized workers/families on board, and he's a big bluff man with a deep voice and a rock-solid demeanor dealing with serious issues: don't underestimate how much better that plays against Reagan than Deputy Dawg Carter
- Edmund Muskie: sick of being in the Senate he seizes his last chance for the big play. Also a budget hawk who was nevertheless a leading environmentalist and friend of the AFL-CIO, a darling of political columnists, and probably spent Ford's second term working very deliberately on his rep as a senate leader.
- Fritz Mondale: seen as the good thing about Carter's run and Hubert Humphrey's protégé, he probably runs. But past Iowa the map is not so great for him to try and get past Carey and Muskie in New England and the Northeast, and the next guy in Florida.
- Reubin Askew: the reforming Southerner who
should've run in 1976 instead of Carter, Askew (and I know people who knew him and have spent decades deep in Florida politics including my father's best friend) was a deeply decent man, a much more charismatic and engaging one than Carter, lived up to his nickname "Reubin the Good" and while he was a social conservative on abortion and (strangely) no-fault divorce he was a flaming liberal on economic equality, civil rights (one of the first pro-civil rights state legislators in Florida, and backed the Equal Rights Amendment to the hilt), and was a successful anti-corruption crusader in one of the most corrupt states in the union. With his personal connections throughout the region he can dominate much of it against the Catholic Yankees Carey and Muskie, enough to make him look like maybe he could win and definitely enough to be indispensable to whoever does win.
- Scoop Jackson: I think he runs, and runs to the right of Ford on the Cold War, but again his map early on is harsh and he may as much as anything be testing the water for the two-spot on the ticket.
- Jerry Brown. Jerry, Jerry, Jerry. He probably gets reelected comfortably in anti-Ford midterms in '78, but for a reforming civil libertarian technocrat governor of the biggest state, he's his own worst enemy. "Governor Moonbeam" was a creation of his famously disastrous 1980 campaign. He needs not to repeat that here but there's a danger he will.
How does all that work out? Could be one of several ways. I think unless he gets the best possible campaign team around him the early map forces Mondale and Jackson out. I think Askew performs great in the south and does decently but not well enough elsewhere. I think Brown likewise has trouble breaking through except with some excited college students in the early states. I think Muskie is a number of people's safe play but he's a brittle and temeramental campaigner not known for organizing well (those things as much as the Canuck letter ended up killing him in '72.) I think it comes down to Carey, Muskie, and Askew, and that until the big run of primaries (led by California) on June 3rd no one has a majority, but Carey could claim the loyalty of "Reagan Democrat" auto workers who trust him to care about their jobs and paychecks
and has the liberal-cultural endorsement of Teddy Kennedy so he's the strongest of the three. I think given the chance to sit down together Askew just
likes Carey the fellow governor better than the intellectual and brisk Muskie. So I think they form a team and it's Carey/Askew '80 on that side. Your mileage may vary.
Reagan picks someone who will complement him (maybe Howard Baker to counteract Askew, maybe Charles Percy to make a play for Illinois' 26 electoral votes, maybe even Connally to go for more Reagan Democrats.) He will run hard and he will be good in debates. There may be some rally round the flag depending on what Ford does or does not screw up in the Persian Gulf. He will run ads that will appeal to "leaners" prejudices and brand the Democratic nominee a tax-and-spend leftist. It will do him some good. But he will get hit with the right-wing things he said in the primaries, by partisan Democratic turnout because twelve years of Watergate and Vietnam and inflation and unemployment and now the Middle East dammit, maybe by a liberal-Republican revolt like Perot shanking Poppy Bush in '92. Mondale will struggle but if he has good people around him he could weather it with a good running mate. Muskie is the most vulnerable and the best chance for Reagan to sneak a win, really the only chance. Carey/Askew are I think moderate enough where they need to be to keep votes they need in the Midwest and upland South that, combined with the other structural factors in their favor (insecurity, inflation, unemployment, creating doubts about how radical Reagan's solutions are) that they can win in the clear. Not by a lot because the country is ever more partisan, but definitely in the clear.
So that gets us to the Eighties (whew!) and Part Two.
