Time’s Jay Carney finds it very peculiar that potential Republican candidates are polling better than Democrats in hypothetical presidential match-ups. After all, he says, President Bush’s approval numbers are low, the war is unpopular, and the Democrats just won the mid-term elections.

Carney offers a few possible explanations, noting the maverick character of the Republican front-runners and the polarizing force of Hillary Clinton. (John Hood, at the Corner, also believes Hillary is the key.)

But more is at play here, I suspect, than the burdens of Hillary’s history. Though the Democrats have some significant tactical advantages, the Republican field is much more presidential in some key respects. The three leading Democrats at the moment are Hillary Clinton, Barack Obama, and John Edwards. All of them are Senators (or in the case of Edwards a former Senator who has been running for President since he left), and the Senate has not produced too many Presidents lately. The last sitting Senator to go to the White House was John Kennedy, 47 years ago. None of the Democrats has a bit of executive experience, and all are also fairly generic down-the-left liberals, even if Obama would prefer to pretend he is not.

The three leading Republicans, meanwhile, are John McCain, Rudy Giuliani, and (though a distant third in most polls) Mitt Romney. One is a Senator, though hardly a generic conservative. Another is a moderately liberal former mayor of a very liberal city, and the third is a conservative former governor of a very liberal state. The latter two have extensive executive experience (though no former mayor has ever been elected President, running New York City might as well count as a governorship.)

Conservatives (like me) are not happy with the absence of a reliable generic conservative in the upper tier of candidates, and recent interest in former Senator Fred Thompson has had a lot to do with that. But the fact is, the Republican field is diverse, interesting, and very experienced, while the Democratic field consists of three liberal Senators whose combined years in office are fewer than John McCain’s alone.

Any of the three leading Republicans might realistically take several of the states Al Gore and John Kerry carried in the last two elections. Of the three Democrats, only Obama stands a plausible chance of stealing any Republican states—his charisma, his positive tone, and (let’s be frank) his race could extend his appeal. But Obama is also the least experienced of all the major candidates.

The supposed Democratic advantage has everything to do with President Bush’s low standing in the polls, and if it stays low well into 2008, it would certainly be a drag on Republicans. But Bush won’t be running in 2008, and anti-Bush venom has been the Democrats’ principal theme since 2001. Deprived of a target for such venom, what platform have they built to run on? And who can they count on to lead them?

Maybe the poll results are not so mysterious.