I first and foremost believe that Martin would jump into the murky waters of Politics unlike his companions, Jesse Jackson, Andrew Young and others did IOTL. He denied an request from Antiwar Activist Dr. Spock to run on a Peace Party platformThe one that that we haven't talked about, is the increasing margilization that Martin was getting from within the black community. Alot of the Baby Boomer African Americans were tired of the ideology of Nonviolence and were openly embracing the words of Huey Newton, Angela Davis and other Panthers who preached Black Nationalisim more in vain with Malcom X rather than King.
RB, is right to say that he was moving toward a focus on Poverty rather than the increasingly militaristic Civil Rights. Most of the members of the Southern Christian Leadership Conference agree that he was planning a "Poor People's Campaign" that never got off the ground. So in a scenario in which he lived, I can see the issue of Poverty be an issue that he would follow for the rest of his life.
I do believe, if he comes out openly dennoucing Bussing in the early Seventies, his relative popularity amongst most Middle Class Americans plumment and he may honestly decide to take a break take a more active role in raisng his kids during the 70's. Or he could take an even more internation profile, dennoucing Apartheid and other Totalitarian regimes.