Newt Gingrich keeps whining about negative ads and is so desperate to stop his slide in the polls that he challenged Mitt Romney to a one-on-one debate. (I’m sure that got a round of laughs in the Romney campaign headquarters.) Rep. Ron Paul (R-Tex.) let his mask of semi-respectability fall when he praised Bradley Manning, accused traitor, and got huffy with Gloria Berger (my gosh, even Sarah Palin hung in there with Katie Couric) when she asked about the racist, conspiracy-filled newsletters sent out under his name in the 1980s and 1990s. Could it be that both may underperform in the Iowa caucuses?

So far, polling evidence suggests Newt Gingrich keeps falling while Ron Paul has yet to dip. But the assaults on Paul have just begun to break through (the CNN episode is deadly since it can be replayed again and again), so it’s not clear whether there is time for the negative images to sink in and what, if any, impact the latest incidents will have on his prospects. At the very least, we can imagine the lid on his support is more secure now that non-Paulites can assess his demeanor and wacky views on national security.

The blows suffered by Gingrich and potentially by Paul raise the same question we’ve had since the rollercoaster presidential race began: Where do the sliding front-runners’ voters go? In the Post-ABC News national poll Romney leads as the second choice of GOP voters with 21 percent. After Gingrich and Paul, Texas Gov. Rick Perry and Rep. Michele Bachmann (R-Minn.) get 11 percent each. In short, the simultaneous decline of Gingrich and Paul would likely benefit Romney most of all.

If Paul falters and doesn’t win the caucuses things become more complicated for the also-ran candidates. Without a Paul “mulligan” (disregard the caucuses’ results since the attendees selected a nut), Perry, Bachmann and Rick Santorum will have to face a harsh verdict if they don’t lift themselves out of single digits and get close to the front-runners. With Gingrich and Paul on the decline, they might have incentive to hang in there, but lack of money and momentum will begin to take their toll.

Romney is now spending heavily and spending time in Iowa. He’s become more visible in TV interviews. And he has no more debates to endure while his opponents hurl anti-Romneycare barbs his way. He now has the opportunity, if not to win, then to come close and thereby preserve momentum heading into New Hampshire. He has been able to stick to his general-election rhetoric and policy approach because of his opponents’ inviting targets. Rather than try to outflank them from the right (probably a losing endeavor), he can hammer them on character, experience, electability and private-sector know-how.

With each passing day, it becomes more difficult to alter the momentum in the race. Those in trouble will find it hard to break out of their rut, and those on the upswing, barring a gaffe or major revelation, are likely to continue their ascent. Soon the race will turn from persuading undecideds to energizing supporters. And that is where the better organized candidates with local support — Romney, Paul, Bachmann and Santorum — have a chance to get those last few voters to caucus for them.

Finally, Iowa may have caught a break with Paul’s errant moments and generally fretting about the Hawkeye state becoming the LaughingStock state. For better or worse, a Ron Paul defeat may be the best argument for Iowa to keep its first-in-the-country contest.