But then again, only Nixon could go to China, which I think is a pretty strong argument.
By 1972 any Republican could have recognized China for the same reasons Nixon did and gotten away with it. A liberal like Rockefeller
may have been attacked from the far right as an "appeaser" because of Goldwater's personal animosity towards him, but the vast majority of the country including most Republicans approved of recognizing China. It was the Democrats who had their hands tied in this regard, since they "lost" China in 1949.
Similarly, only Reagan could get the INF Treaty regarding intermediate range missiles in Europe, and esp. shepherd it through Senate ratification.
This I seriously doubt. Detente had bipartisan support since 1972, and all the major arms treaties that Presidents submitted to Congress (SALT 1 & 2, START 1) were passed.
And only Clinton could get welfare reform.
Any Republican President would've signed welfare reform into law. It wasn't Clinton's initiative remember, it was that of the Republican Congress which passed it three times. The first two times Clinton vetoed it, but under political pressure he buckled and reluctantly signed the third bill into law in 1996.
Anyway, you do have a broader point that being a conservative McCain would have some political cover to punish banks and with this I agree. Certainly more than Obama would, which ties into the whole "only Nixon could go to China" trope. (Though as I pointed out above it's more accurate to say that "only a Republican could go to China"). But by 2009 America was well to the right politically of where it was in 1972. If a Republican President tried to do anything Nixon did (price controls, create new government agencies like the EPA, etc) then they would be the subject of furious hatred from the conservative base and would face a primary challenge in 2012. Whatever political cover McCain has by being a Republican would be outweighed by the full force of the conservative movement's opposition to him for "betraying" the GOP, Reagan's legacy, and the American people. (This may sound hysterical, that's because it is. But that's where the GOP was at by 2010 and 2008 is too late a POD to stop that...).