A poor performance in February's voting -- the Iowa and Nevada caucuses and the New Hampshire and South Carolina primaries -- could force out some of the weaker candidates.
Often the story about the losers in campaigns tells us a lot about the dynamics of a contest and the reasons that other candidates are succeeding at a particular moment.
So it is worth asking -- what has happened to these candidates? Why are they in political trouble? What is going wrong?
Jeb Bush: The onetime front-runner is anything but that. The former Florida governor has suffered one of the most dramatic falls from grace that we have seen in some time. When the campaign started, he was widely believed to be a candidate who would be almost impossible to defeat, someone with all the money and endorsements necessary to cruise to victory.
But the situation has turned out very differently. His biggest failing has turned out to be his inability to connect on the campaign trail and in media appearances. He has languished in front of crowds, having trouble articulating his reason for wanting to be in the race and performing with the kind of energy level (famously lampooned by Trump) that leaves many crowds eagerly waiting for the next candidate to take the stage.
In last week's debate, he scored points with some one-liners aimed at Trump -- "You can't insult your way to the presidency" -- and could gain from being a candidate willing to take on the bombastic front-runner. But will he see a lasting gain in the polls?
Bush seems to be someone who is running at the exact wrong time. He continues to struggle with the controversy over his brother's decision to go to war in Iraq. He boasts of experience at a moment GOP primary voters want to tear down the establishment. That anger has undercut many of the benefits that his experience in government could have brought Bush in another year.
A surprising fade -- and an expected one
Rand Paul: Many observers once thought that Rand Paul could be the most exciting candidate in the room. He was young, he espoused a kind of libertarian philosophy (though libertarians argued it was anything but pure) that was exciting to many younger Republicans, and he had eclectic views on issues like criminal justice that made it difficult to easily pin him as a Republican right-wing stalwart.
The campaign has not turned out so well. He has demonstrated a number of personal failings, such as a fierce temper as well as hostile interaction with female reporters that, early on, damaged him in interviews.
Perhaps more important is the fact that his skeptical stance about an aggressive foreign policy was out of step with so many Republicans. The truth is that much of the party has embraced the hawkish outlook that has prevailed among Republicans since 9/11. There is little room for Republicans outside of this fold. Even his attacks on surveillance, which had gained some hold, lost traction as the threat of ISIS has become clearer.
Paul's efforts to reach across party lines on issues like criminal justice have some appeal with younger voters, but not with the majority of primary voters who are older and not as sympathetic to these kinds of questions. At last week's debate, he scored some points by criticizing Chris Christie when he said: "If you are in favor of World War III, you have your candidate." The problem for Paul is that right now there are many Republicans who like militaristic rhetoric in today's troubling times.
Speaking softly doesn't work
Lindsey Graham: The struggles of the South Carolina senator, who became the latest to drop out of the race Monday, are much less surprising than those of Jeb Bush.
Graham remains a favorite conservative in the minds of many of his colleagues because of a number of attractive qualities -- his long record as a hawk in an era of great national security challenges and political fear, and his roots in the regional bedrock of modern conservatism: the South.
He has a good sense of humor that he knows how to deploy politically, even with delicate issues. Last week, in the early bird debate, he said: "Donald Trump has done the one thing you cannot do: Declare war on Islam itself. ISIL would be dancing in the streets, but they don't like dancing."
Beyond his quotable lines in the debates, Graham's candidacy barely attracted any attention. Graham's low-key, folksy style didn't play well in the modern media age of cutthroat, smash-mouth politics. Speaking softly and carrying a big stick don't really work so well in the era of Twitter and cable news.
Graham also prides himself on knowing how to work within the system and form ties to fellow members of Congress. But since comity is out and partisan warfare is in, these virtues don't really sit well with many of the GOP's primary voters. His hawkish credentials have also been echoed by most of the other candidates so there was not really much to distinguish him.
Governors weaker than in past cycles
Mike Huckabee rose to fame in Arkansas at a moment when the religious right played a big role in partisan politics shaped by the culture wars. As the agenda has turned to questions like immigration, terrorism, and climate change, those concerns have not been front and center.
Ironically, even in this anti-Washington environment, the other governors and former governors who are running for office are not having an easy time. In past decades the role of governor has been an attractive position for running for president -- governors get to display executive leadership without becoming enmeshed in the politics of the Beltway. Yet this time they have had trouble claiming true outsider status, especially in contrast to Trump and Ben Carson. While some of them, like Bush, Christie and John Kasich are hanging in there, others are unlikely to survive much longer. George Pataki, a conservative from the blue state of New York, was one of those executives who liked to boast of how well he could govern in years where northeastern Republicans still had some role within the party. But those times are gone, and his low-key and at times dull public presence is not really made for the national stage.
Former Virginia Gov. Jim Gilmore, who comes from a critical state for the GOP, has barely made a blip on the political radar. His campaign has fizzled from the moment he started, lacking definition and lacking sizzle. Christie's record in New Jersey, from the problems of the economy to the bridge scandal, has continued to be a black mark on his campaign, though his focus on campaigning in New Hampshire has helped him in the first primary state.
Not registering in the polls, or a flash in the pan?
Rick Santorum: His time came in 2012 when he shocked Mitt Romney early on with his strong support among blue-collar conservatives. Santorum was one of the first Republicans to tap into the anti-establishment anger Republicans directed against Washington and Wall Street. But now his themes are being repeated by a number of candidates, including Trump. His thunder seems less distinct in 2016, while his staunchly conservative views on same-sex marriage remain a huge liability in the current environment.
Ben Carson: Carson is much more like the flash-in-the-pan candidate that most experts thought Trump would be. Though Carson had a few weeks where his low-key personality and eclectic views attracted some interest and a spike in the polls, his inexperience and tenuous command of foreign policy became a huge liability as the agenda shifted to foreign policy.
Carly Fiorina: Despite a few strong debate performances, her campaign has never really taken off. She has been dogged by questions about her business record, which has continued to provide fodder for her opponents. Her highly conservative views have undercut some of her promise to broaden the Republican electorate while she has not had much success defining what her candidacy is really about, other than theoretically being the best person to handle the "gender" problem the GOP candidate would face if running against Hillary Clinton.
The candidates are trying to stay in the race as long as possible, hoping that the early polling isn't really accurately predicting how the voters will choose. It is also important to remember that according to all the best social science research, the "invisible primary" of fund-raising and endorsements matters more than national polling at this stage, so some candidates believe all of the talk of inevitability for Trump or others at the top of the polls is misplaced.
But staying around will be hard, especially if voters don't flock to their sides and if their campaign coffers are depleted. Many of these candidates could join Lindsey Graham, Rick Perry, Scott Walker, and Bobby Jindal in leaving the stage. We will soon gain a better sense of what the real contest is in the major primaries, and of whether the party can unite around a candidate -- or will have to choose one at a messy convention.