Zell Miller was a Democratic Senator from Georgia, who had fiery denunciations of the Democrats back in 2004; he backed George W Bush, and even spoke at the Republican Convention.
But for a moderate Republican like John McCain, we have a more subdued version: Senator Joseph Lieberman (link). FYI, Senator Lieberman was elected as an Independent after losing in the Democratic primaries to Ned Lamont, an anti-war businessman and a darling of leftist bloggers.
I don't like Senator Lieberman, because in 2000, as Al Gore's running mate, he continued to run for the Senate. I firmly believe that if one competes for the highest (or second-highest) office in the land, there should be a 100% commitment to that effort, with no other distractions. But Senator Lieberman backs Senator McCain for much the same reason I did - finishing the war in Iraq. Then again, that doesn't make Senator Lieberman unique in my world!