Thompson's biggest disadvantage may be that he is entering the race rather late, as things seem to be shaping up this year. Many of the big donors, and most of the knowledgeable campaign managers, have already been signed up by one or another of the candidates who have preceded him into the race.
But in another respect his timing is impeccable. The millions of conservative voters who constitute the Republican Party's base, and whose support is going to be indispensable to any nominee who hopes to win the election, have had visible difficulty generating great enthusiasm for any of the other candidates. Giuliani is an outspoken liberal on a good many important issues. McCain is a bad-tempered maverick who has been all over the map. Romney is a latecomer to various conservative causes, and anyway he's a Mormon. But Thompson is a loveable natural-born conservative without a flyspeck on his record. If he wades into the race, he may excite conservatives as much as Obama excites liberals, political journalists and war protesters.
And by the way, if his critics try to dismiss him as just (or mostly) "an actor," don't forget what happened the last time they tried that.