The 5 Republicans who could be president

We began with more than 20 Republican candidates. Seventeen made it to a formal announcement. Eleven reached Iowa. Now six remain; and with former surgeon Ben Carson going nowhere, only five have a chance to win the nomination. Here is how each of them could do that.

• Billionaire Donald Trump wins by repeating what he did in New Hampshire. As long as the rest of the field is split, he'll benefit in two ways: Negative ads will be aimed at other Republicans, and a third of the vote will be enough to win.

It remains an unlikely path. Losers drop out. Before long Trump will probably have only one or two opponents. This is bad news for a candidate who remains unpopular among many Republicans and appeared vulnerable to negative ads in Iowa. His ability to dominate the media has been his greatest strength, but that's more difficult now than it was before Iowa, and it will continue to get harder.

• Texas Sen. Ted Cruz wins by surviving as other candidates wash out, then by defeating Trump head to head. Rubio's setback in New Hampshire helps him. If Cruz can crush the Florida senator and the other mainstream conservatives in South Carolina, he'll be in good shape whether he beats Trump there or not.

The challenge for Cruz is his unproven ability to draw votes from the ideological center of the Republican Party and his prospects for dominating groups most likely to support him. He has a fair amount of support from party actors, but it is factional. And it can't help that so many Republicans who have worked with him can't stand the guy.

• Florida Sen. Marco Rubio wins if his post-debate collapse in New Hampshire turns out to be short term. He had a large lead over Jeb Bush and John Kasich nationally before Tuesday and in South Carolina in January, the last time its GOP voters were polled. If that holds, he'll knock both Bush and Kasich out soon, and he remains the logical destination for most of their voters. That's the same path he's been on all along: survive the early events, and appeal to the widest range of groups, eventually consolidating all of the anti-Trump and-or anti-Cruz vote.

He remains the most likely nominee right now, even though he has lost a lot of leeway. Another bad debate performance could doom him, as might anything that reinforces a view of him as an empty suit. Most party actors are still sitting on their hands. Still, Rubio has more support from them than anyone else in the race, and that backing is not just from members of Congress. He now has a large lead in support from current and former state legislators. He has added about 100 endorsements from those state-level politicians in the last three weeks, and now has 324 current and former state legislators backing him; Cruz has picked about 50 in this group over the same period and is second at 244. Bush (185), Kasich (141) and Trump (37) haven't been adding to their support at all.

• Former Florida Gov. Jeb Bush wins if he surges off of his mediocre fourth-place finish in New Hampshire. His path to victory is basically the same as Rubio's. He moves up to a strong third (behind Cruz and Trump) or better in South Carolina and knocks Rubio and Kasich out, then consolidates anti-Trump and anti-Cruz votes.

He faces several hurdles that make success unlikely. We have little polling this week, but Bush still probably trails Rubio in South Carolina and nationally. If the press touts one candidate for his showing in New Hampshire (besides Trump), it's more likely to be Kasich than Bush. Republican voters just haven't liked Bush much so far, even though he's outspent everyone. Also a problem for Bush: In a three-way contest with Trump and Cruz, his more moderate image and positions would make it tougher for him to compete for conservative and very conservative votes (in contrast to Rubio, who has positioned himself as more conservative).

• Ohio Gov. John Kasich wins if his second-place finish in New Hampshire generates plenty of press attention, producing a surge of voter support, which in turn allows him to beat Rubio, Bush and Cruz in the coming contests. But he would need to do well even in conservative states, beginning in South Carolina. He can't win the nomination without that support.

A big theme of this election cycle is that any candidate can get a sudden surge. But so far there's no sign the Ohio governor is receiving one. Even if this happens, Kasich may just be too moderate to win. We'll know more when we get some post- New Hampshire polling. Kasich's campaign has talked about hanging on until some big Midwestern states vote in March, but that's not a winning path.

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Jonathan Bernstein, a politicial scientist and author, is a Bloomberg View columnist.