John McCain’s victory in the Florida primary, coupled with Rudy Giuliani’s distant third-place finish and subsequent withdrawal from the race, clarifies the previously murky Republican presidential contest. McCain is now the clear frontrunner to win the GOP nomination. With Giuliani out, Romney is the only one who can stop him.

Giuliani was an unorthodox candidate, who pursued an unorthodox campaign strategy. Winning the Republican nomination is a state-by-state affair, not a national referendum. Giuliani’s many months atop the national polls meant little, as he was unable to win—or even place second—in any of the first six state contests. His name is now the answer to the question, “What do you call someone who finishes 6th, 4th, 6th, 6th, 6th, and 3rd?” It is possible that if Giuliani had competed more aggressively in the early states and lost, his exit from the race would have come sooner. But what is true of the lottery was true for Giuliani: you have to play to win. And because Giuliani allowed the other candidates to claim the spotlight in the early races, his previously robust support in Florida dwindled.

The reason that Giuliani feared competing in New Hampshire and subsequent states was the resurgence of John McCain. With McCain down and nearly out last fall, Giuliani seemed attractive to independents, moderates, and national security hawks. But a competitive McCain hurt Giuliani among all of these voters.

McCain’s win in Florida does not end the GOP race, but it makes Romney’s challenge that much harder. The former Massachusetts governor has two big strengths: he has the resources to run nationwide, and he is the leading conservative candidate. His money will certainly be helpful on “Super Tuesday,” February 5th, when he will far outspend McCain. But given the large number of contests that day, Romney’s flush war chest will be spread thin.

McCain is able to compete nationwide, while Mitt Romney must effectively concede certain regions of the country.

As for his conservatism, Romney has made enormous efforts to become the consensus conservative candidate. In a normal Republican primary race, the conservative beats the moderate. Yet while Romney has won many conservative backers, he has not yet consolidated the support of southerners and evangelicals. If Romney could unite his base with Mike Huckabee’s, he would be a formidable candidate. But that is not likely to happen in the short period of time before February 5th.

McCain is also not the traditional moderate. Despite his record of breaking with many Republicans on such issues as tax cuts, immigration, and campaign finance reform, McCain takes relative conservative stances on most social issues. More importantly, his military service, heroism, and foreign policy experience wins him the support of many veterans and defense hawks.

All of this means that McCain is able to compete nationwide, while Romney must effectively concede certain regions of the country. In the South, Alabama, Arkansas, Georgia, and Tennessee will vote on Super Tuesday. In each of these states it is a McCain-Huckabee race, and the 188 delegates—out of 890 delegates selected overall on February 5th—can be split among several candidates, depending on their electoral strength statewide and in the various congressional districts. The most likely result is that McCain and Huckabee will take most of the delegates, leaving only a handful for Romney.

Before Giuliani’s exit from the race, a McCain-Giuliani contest was shaping up in the Northeast, with polls showing a McCain lead in many states. With Giuliani out, the Arizona senator is likely to sweep New York, New Jersey, Connecticut, and Delaware. Those states are winner-take-all, so McCain is expected to pocket all of their combined 184 delegates. McCain is even competitive in Romney’s home state of Massachusetts, which assigns its 40 delegates on a proportional basis. Romney and McCain are likely to split those delegates relatively evenly.

California is the biggest prize, and McCain leads in most polls. But even a Romney win in California would not be decisive, since almost all of the Golden State’s 173 delegates go to the winners in each of the congressional districts, which are likely to be split between McCain and Romney.

While McCain is competitive nearly everywhere, Romney is dominant only in winner-take-all Utah (36 delegates) and perhaps in a couple of western caucuses.

The best Romney can hope for is a muddled result that keeps the delegate count close. Romney needs Huckabee to beat McCain decisively in the South to keep McCain’s delegate yield there low. If Romney then picked up the few western caucuses and primaries, did well in Illinois, and competed in the southern border states of Oklahoma and Missouri, he might be able to claim that February 5th was not decisive. And he would have the resources to go on battling McCain state-by-state to the end.

But right now, this scenario appears unlikely—which shows just how close McCain is to grabbing a stranglehold on the Republican presidential nomination.

John C. Fortier is a research fellow at the American Enterprise Institute.