Thompson only endorsed McCain after Romney dropped out and it became clear the McCain has the nomination sowed up:
http://blog.washingtonpost.com/the-trail/2008/02/08/fred_thompson_backs_mccain.html
He's only trying to unify the party now that the nominating contest is effectively over.
As to Thompson's endorsement of McCain in 2000, that was 8 years ago and McCain was in many ways a different man back then. He hadn't shown his ugly side and come out for an illegal alien amnesty yet. Nor had McCain had a chance to oppose the Bush tax cuts yet. In addition, Bush was spouting that "compassionate" conservative nonsense that made him sound like a leftist, and we all know how that turned out: the single biggest expansion of government since the great society. One of the (few) good things McCain did was vote against Bush's prescription drug giveaway to geezers.
In many ways, the 2000 nominating contest was worse than 2008 for conservatives; this year we had two good choices (Romney and Thompson). In 2000, there were none, and it wasn't at all clear (it still isn't) that Bush was a better choice than McCain.
Oh, and one last thing: if you think the Huckster still has a snowball's chance in hell to get the nomination, I have a bridge to sell you. The only reason the "Christian" socialist is still in it is to get the Veep spot. I pray every day he doesn't.
“In many ways, the 2000 nominating contest was worse than 2008 for conservatives; this year we had two good choices (Romney and Thompson). In 2000, there were none, and it wasn’t at all clear (it still isn’t) that Bush was a better choice than McCain.”
Another irony: McCain would have been not too different from Bush on things except spending, and right about now a different Bush - Jeb - would be tanned, rested and ready for the Presidency.
The smarter and more conservative one ... kind of like Romney (who’d in turn get no notice Jeb Bush were around, and Fred Thompson would also be unneeded if George Allen was around too to be contender).
In the end, this was McCain’s own fault for getting a come-uppance from saying rude things about Christian conservative leaders in 2000. History has a way of turning on a dime.