You would never think that George W. Bush has more than three years left in office by watching the Democrats who are already maneuvering to succeed him. Of course, when it comes to galvanizing the opposition, there’s nothing like a president in the polling dumps with no obvious successor from his own party.

But there is another reason the Democrats are busily preparing their White House battlefield. They already have a clear front-runner, forcing the also-rans to rev their engines for attention — and show whether they really have “what it takes” to overtake the leader.

On the surface, Sen. Hillary Rodham Clinton of New York is doing the least to signal interest in a 2008 race. She and her aides insist that they are only focused on her November 2006 re-election campaign. She is so careful to avoid the appearance of presidential ambitions that she takes pains to steer clear of New Hampshire, site of the first-in-the-nation primary. She hasn’t been there for nearly a decade, and recently opted to fly between Maine and Massachusetts fundraisers, bypassing a two-hour drive that would have included 14 miles through the Granite State.

Her potential rivals are not so shy, giving the early-contest states — Iowa and New Hampshire – plenty of action. They are pursuing top Democratic contributors and consultants, burnishing their resumes, compartmentalizing their weaknesses and positioning themselves on issues of the day, such as the war in Iraq.

Mark Warner, who is wrapping up a successful tenure as Virginia’s governor, made the obligatory trip to New Hampshire, with more to come. Gov. Bill Richardson of New Mexico recently moved to clear away the embarrassment of padding his r??sum?? with spurious claims that he was once a major league baseball pick. The party’s 2004 nominee, Sen. John Kerry of Massachusetts, issues daily talking points in a desperate effort to matter. His running mate, former Sen. John Edwards of North Carolina, is feverishly on the stump.

For her part, Clinton is circulating the country raising money for Democrats — this month alone in Colorado, Illinois, Kentucky, Michigan and New Jersey. In Louisville on Dec. 2, she boosted party coffers by $600,000 in a state that her husband won in 1992 but struggled to keep in 1996.

Seeking to further establish her national security credentials, Clinton has finally begun detailing her thoughts on Iraq, although the results were murky. She seems to be saying it was a mistake to vote in favor of authorizing the president to go to war, but it would be a mistake to leave.

Still, the threshold question for the Democratic presidential field is whether the nomination already belongs to Clinton. She is “the prohibitive favorite,” one of her rivals, Sen. Joseph R. Biden Jr. of Delaware, said in a recent television/radio interview.

Yet a number of state and local party leaders worry that Clinton is so polarizing and unpopular among independents that nominating her would be a walk off the plank in a general election.

“We can’t win with Hillary as our nominee, partly because the country might not be ready for a woman president, but mostly because that woman would be Hillary,” said an Alabama Democrat who supports Edwards for the top spot in 2008. This activist, like others I’ve spoken to around the country, did not want to be named for fear of retribution in case Clinton does win the nomination.

The fear of going negative against Clinton in public also grips those who would run against her. None of her challengers show any inclination to explicitly make the charge that she can’t win against the Republican nominee. For starters, doing so would probably end any possibility of becoming Clinton’s running mate.

That’s my test for separating serious rivals from those who are just auditioning to join a Clinton ticket. Those who can’t find a way to publicly speak to party concerns about Clinton’s general-election appeal will not be serious presidential contenders in my book.

Instead of taking on New York’s junior senator, the rest of the Democratic field is moving forward with campaigns that seem to be an exercise in collective denial, pretending she isn’t there. And yet, if she is to be denied the nomination, her rivals should go negative early, before she has raised so much money and lined up so many endorsements that no one can take her down.

Edwards is my bet to be the first credible 2008 contestant to go after Clinton. Surely he has no interest in playing second banana a second time, giving him every incentive to antagonize her. And he recently showed a penchant for plain talk, announcing unequivocally that he made a mistake in voting for the Iraq war resolution.

Such plain talk about Clinton’s 2008 chances against a Republican would be the smartest maneuver that any Democrat could muster at this early stage. Let’s wait and see if anyone has what it takes.

Contributing Editor Craig Crawford is a news analyst for MSNBC, CNBC and “The Early Show” on CBS. He can be reached at ccrawford@cq.com.