Tonight's Democratic show-down in Las Vegas features one definite heavyweight fight, set up by the press as Hillary Clinton versus some combination of Barack Obama and John Edwards. We noted from Philadelphia that many were using boxing metaphors, and what better venue than right off the Strip, where the biggest bouts are fought, to continue that metaphor?

But pay attention to what is becoming an increasingly heated, if subtle, undercard. That fight, between Obama and Edwards, is a contest for the half of Democratic voters who haven't already said they would back Clinton. Both argue that they draw the clearest contrast with Clinton, and both shy away from taking on each other. But this fight is perhaps more urgent than a battle against Clinton: United against her, her opponents might stand. Divided, they will probably fall.

The battleground where an Obama-Edwards grudge match will be fought is Iowa. But unlike previous years, strategists for both campaigns say, this year there might be only two tickets out of the state, not the traditional three. "Whoever comes in third here is going to be in bad shape," Edwards strategist Joe Trippi told Politics Nation in Des Moines. That candidate, he said, will be "on life support." And the race is shaping up, in his mind, as a perfect opportunity for Edwards. Obama, he argues, "has had ten months with the whole world saying, 'It's between him and her,' to make it between him and her. Guess what? He's failed at that."

Clinton's problem, says Trippi, is that much of her support is reluctant, as opposed to enthusiastic. That reluctance presents an opening for a new candidate to emerge, and when that happens, "the race resets. And when it resets, she's going to lose a lot of her support."

The media's intense focus gave Obama an opening that Trippi says he missed. "There should have been a way to leverage that [media] focus, you know, to turn the race that way," he said. "People have looked at Barack Obama and have made a decision about him."

Obama backer David Axelrod thinks the subtle digs at his candidate, from the Edwards team, are just beginning. "Obviously, I think [Edwards is] a very, very determined guy. This is his second shot," Axelrod said at an Obama event in Chariton, Iowa. "Ultimately, [Obama's] quarrel is with a style of politics that has come to characterize Washington," he said, and those who would "shift and dodge, and wind up where you need to get to in the short run."

"People look for authenticity," Axelrod said. "They look for consistency. I think they're looking for people to stand on long-held principles and not on sort of short-term calculated posturing." Though the implicit shot touched some of Edwards' own inconsistencies, the Obama strategist couldn't help but return to his main target: "That's, I think, one of the reasons why Senator Clinton has run into some problems."

At the Philadelphia debate, though, it was Edwards who got credit for being fastest on the attack and surest of foot in drawing contrasts. The clarity of those contrasts are key, Trippi said. "We're going out every day and making sure people understand that the clearest choice in this race is between Hillary Clinton and John Edwards."

Obama, who has looked less steady going after his rivals, is only warming up, according to Axelrod. "[Obama] is happy to, and willing to, respond to any challenge. That's been true throughout his political life," said the strategist who cut his teeth in rough and tumble Chicago political circles. "He comes from a pretty tough political arena."

The race for the Anybody But Clinton crowd is not a race for second place. With a slim lead, if that, in Iowa and a not-insurmountable lead in New Hampshire, the presumed front-runner has her work cut out to reach the nomination. But as Edwards and Obama continue to aim fire at Clinton, the clock is ticking for them to make contrasts with each other. Tonight, the undercard is just as important as the main event.