Analysis

As Mitt Romney seeks to close out the Republican presidential race, momentum is facing off against two time-honored rivals: ego and arithmetic.

Romney's sweeping win in Nevada gives him his third convincing victory in the first five states to vote. With no one else able to boast of Romney's financial advantage and campaign infrastructure, precious few opportunities remain for his rivals to gain a foothold that would knock him off course.

But this is where delegate math is not Romney's friend, not this year. The proportional allocation of delegates - as opposed to the winner-take-all format that dominated previous cycles - combines with a back-loaded calendar to leave virtually no chance for Romney to end the race quickly, unless his rivals cooperate.

As of today, only 143 delegates have been awarded - barely 6 percent of the full complement of 2,286 who will be selected to cast ballots at the Republican National Convention. Fewer than 200 additional delegates are up for grabs through the remainder of February.

And Super Tuesday isn't half as supersized as it was four years ago. At the end of the night March 6, only 820 of the 2286 delegates will have been awarded - less than 36 percent of the full total.

What's more, the Super Tuesday states will be allocating delegates proportionally. The inclusion of several big southern states on that date would make a Romney sweep unlikely in any event.

That means the only way to end the race early is for other candidates to drop out quickly. Ron Paul has already made clear that his main goal is compiling delegates, as opposed to actually challenging for the nomination, so the Texas congressman isn't going anywhere before Tampa.

Rick Santorum remains the most closely watched to drop out, having failed to recreate any of his Iowa magic. But he has a real shot at winning or running a close second in Minnesota's conservative-leaning caucuses Tuesday, and has a clear shot at Romney in Missouri that day. (Delegates won't be awarded in the Missouri primary on Tuesday, but Gingrich's failure to qualify for that ballot means an opportunity for Santorum to make his case in a head-to-head matchup.)

That leaves Newt Gingrich, almost certainly Romney's strongest challenger, but not a candidate with a consistent message day to day, or a strategy for winning states, as opposed to racking up second-place finishes.

After Nevada delivered another blow to his campaign, Gingrich reiterated his vow to soldier on through the convention. He did not sound like a candidate who's even considering dropping out.

"The vast majority of Republicans across the country are going to want an alternative to a Massachusetts moderate who has in his career been pro-abortion, pro-gun control, pro-tax increase, and who ranked third from the bottom in creating jobs in the four years he was governor," Gingrich said. "So I suspect this debate will continue for a long time."

There is, of course, another factor looming over the GOP primary race: the general election. President Obama's prospects for reelection are looking stronger the longer the Republicans fight it out, with positive economic news combining with the bruising of the Republican contenders to give Democrats a reason to be optimistic.

Things will look much different the day after Romney - or, in the longshot scenarios, Gingrich or Santorum - effectively clinches the nomination.

But that day looks as far off as ever, even as Romney starts to rack up victories that leave him overwhelmingly likely to be Obama's opponent in the fall.