The battle for the Republican presidential nomination underwent a major transformation last weekend when Fred Thompson told Chris Wallace of the Fox News Channel that he is considering entering the race. This is no minor development. Bob Beckel, Clinton's longtime press secretary and now a Democratic commentator for Fox, promptly asserted that Thompson is the only possible Republican contender "who scares me," and he is right to worry.

Thompson first attracted national notice as the Republican minority counsel in Congress's investigation of the Watergate scandal. Television viewers liked the imperturbable figure they saw, and parts in various Hollywood movies came Thompson's way. Then, in 1994, Thompson was elected to the U.S. Senate in a landslide to fill the remainder of Al Gore's term, which Gore had vacated on his election as vice president in 1992. Thompson was elected to a full term in 1996, and served as chairman of the Senate's Governmental Affairs Committee.

In 2002, Thompson's prospects for re-election were rosy but, perhaps understandably, he opted to retire from the Senate and earn substantially more money for himself and his family as an actor. He was promptly snapped up by the wildly popular television series "Law & Order," which cast him as the wise and avuncular district attorney who oversaw its criminal prosecutions.

And recently, the Bush Justice Department enlisted him to accompany Supreme Court nominees John Roberts and Samuel Alito on their visits to key senators when their nominations were up for ratification. The counsel of a shrewd former senator was judged of high value to the nominees.

Thompson has not yet officially thrown his hat into the 2008 ring, but his statement to Fox News was clearly intended to call attention to the possibility, and test the waters.

It is a major development because Thompson has so many undeniable qualifications for the nomination. First and foremost, he is a true-blue conservative, comfortable with all the positions on social issues (abortion, gay rights, gun control, etc.) that give Rudy Giuliani so much difficulty and that have inspired John McCain and Mitt Romney to "flip-flop" in recent years to curry favor with social conservatives. In the second place, he is (as his television career demonstrates) an immensely attractive personality at 64, with a rumpled and thoughtful charm. Thirdly, his service for eight years in the U.S. Senate (four times Barack Obama's current tenure) attests to his success as a political leader. And finally, he hails from a border state -- Tennessee -- with all that implies for electability in the South and elsewhere.

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.