An old post of mine:
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"Ford gestured toward making Reagan his running mate. Reagan responded diffidently, or perhaps coyly. James Baker, who later worked closely with Reagan when he was president, thought a Ford-Reagan ticket would have been appealing to voters and could have happened had either side been a bit more forthcoming. 'You know, Mr. President,' he told Reagan, 'if President Ford had asked you to run with him, he would have won.' Baker added, thinking of what happened in the next four years, 'And you might never have been president.'
"''You’re right,' Reagan responded. 'If he had asked, I’d have felt duty-bound to run.'
"Baker continued: 'President Ford didn’t ask you because we received word from your campaign that you would join him for a unity meeting only on condition that he wouldn’t offer you the vice presidency. And besides that, you very publicly shut down the movement by your supporters in Kansas City to draft you for the vice presidential nomination.'
"'Look, I really did not want to be vice president, and I said so at the time,' Reagan responded. 'But I don’t have any recollection of telling anyone to pass a message to President Ford not to offer me the spot. If he had asked, I would have felt duty-bound to say yes.'
"Baker could hardly believe what he was hearing. “I was shocked,” he recalled. “How different history might have been. Given the intensity of their primary battle, Ford really didn’t want Reagan as his running mate, but the president might have asked if he had thought Reagan would accept. And with a Ford-Reagan ticket in 1976, I think two portraits might be missing from the White House walls today—those of Jimmy Carter and Ronald Reagan.'"
http://www.historynet.com/ronald-reagan-gop-nomination.htm
I am not so sure that a Ford-Reagan ticket would have won. It would have strengthened the ticket in the West and South, but Ford-Dole swept the West anyway (except Hawaii, where I doubt Reagan would have helped) and in the South, Carter's appeal to regional pride was strong enough that the only state Carter carried really narrowly (he lost Virginia) was Mississippi. Still, it is just possible that Reagan would have energized enough conservatives to enable the ticket to carry Ohio as well as Mississippi (though note that Reagan barely defeated Carter in Mississippi even in 1980!). And there is more than a slight possibility that the Ford administration would be so unpopular by 1980 and the GOP brand so tarnished after twelve years in office that no Republican including Reagan could win that year...
(Also, just because Reagan said years later that he would have felt duty-bound to accept does not *necessarily* mean he would have done so in 1976 IMO.)