Jeb Bush’s decision to drop out of the presidential race after a dismal fourth-place finish in South Carolina sped up a process that Republican elites have long been praying for: a winnowing of the field that could thwart the candidacy of Donald Trump.

The theory is that Trump, who notched his second consecutive primary victory on Saturday, is a factional candidate with a “hard ceiling” of support limited to one-third of the party. While that has propelled him to victory in crowded field in New Hampshire and South Carolina, if other candidates quit, argue some anti-Trump conservatives, those voters will consolidate behind an alternative and soundly defeat the blustery billionaire for the nomination.

Trump, however, bristled at that argument during his victory speech Saturday night.

"A number of the pundits said, 'Well, if a couple of the other candidates dropped out, if you add their scores together it's going to equal Trump,'" he said in a mocking tone. "But these geniuses—they don't understand that as people drop out I'm going to get a lot of those votes also. You don't just add them together."

Trump has a point, and a close examination of Republican voter data shows that the "winnowing" theory has four serious flaws.

1. It’s unclear Trump loses a three-person race

An Economist/YouGov national survey released last week tested the theory that Trump would suffer in a three-person race with his two chief rivals. It found Trump winning with 46 percent of the vote, ahead of Marco Rubio with 28 percent and Ted Cruz with 26 percent.

Republican strategist Patrick Ruffini said the poll called into question the theory.

Twitter: Patrick Ruffini on Twitter

Corroborating that finding, an NBC/Wall Street Journal poll in January found Trump leading with 40 percent to Cruz’s 31 percent and Rubio’s 26 percent.

Another survey, by the Democratic firm Public Policy Polling, that was released in early February offered a modicum of hope for the winnowing theory. It found Rubio leading narrowly with 34 percent to Trump’s 33 percent and Cruz’s 25 percent in a three-candidate field.

With Rubio and Cruz well-funded for the long haul, there is scant expectation that the current field will narrow to fewer than three candidates very soon. But even a two-person race is not quite guaranteed to produce a Trump defeat. The NBC/Wall Street Journal poll found Trump losing to Cruz by a margin of 43 to 51 percent in a head-to-head match, but leading Rubio by a margin of 52 to 45 percent.

Cruz and Rubio have sparred in recent weeks over who is better positioned to beat Trump once the field shrinks further. 

"I'm the only candidate who can beat Donald Trump," Cruz told reporters Monday before a rally in Las Vegas, referring to his victory in Iowa.

2. Trump's 'hard ceiling' is overrated

The Cruz and Rubio campaigns have, in recent days, made the case that Trump has run up against his polling ceiling. 

"Donald Trump’s electoral ceiling is in the mid 30s," Rubio campaign manager Terry Sullivan wrote in a memo distributed on Sunday after the South Carolina primary. "Simply put, Donald Trump can never get to 50 percent and only will continue as a front-runner as long as the field is crowded."

On Monday in Las Vegas, Cruz concurred. 

"Donald Trump is a candidate unlike any we’ve ever seen before. He has a passionate, committed base of supporters, but he’s got a ceiling—between 60 and 70 percent of Republican primary voters recognize that Donald is not the best candidate to go head-to-head with Hillary Clinton," Cruz said.

But it's far from clear that Trump has a hard ceiling. Some Republican operatives who have no love for Trump, such as Ruffini and Stuart Stevens, a strategist for Mitt Romney's 2012 campaign, have cast doubt on the "ceiling" theory.

Twitter: stuart stevens on Twitter

One way to test this, pollsters say, is to gauge what percentage of voters could see themselves supporting a candidate.

The January NBC/Wall Street Journal survey found that 65 percent of likely Republican voters could see themselves supporting Trump, a staggering jump from the 23 percent of voters who did last March, before he announced his presidential run. Cruz and Rubio fared modestly better, at 71 percent and 67 percent, respectively.

“The longer Donald Trump stays in the race, the more likely GOP voters are willing to vote for him,” Republican pollster Frank Luntz tweeted in response to that statistic.

By contrast, in January 2012, 59 percent of Republicans saw Romney, who went on to win the nomination, as "acceptable," according to Gallup.

3. Trump’s support is broad-based in the party

While Rubio pitches himself as best-positioned to unite the party, Trump has a case of his own to make. Exit polls in the first three states show strong support for the New York billionaire across age groups, sexes, ideologies, income level, religious inclinations, issue preferences and candidate qualities.

Though he lost some subgroups in South Carolina—like well-educated voters, who Rubio won, and very conservative voters, who Cruz won—exit polls showed no glaring vulnerability that could undermine him. The only GOP faction that overwhelmingly views Trump as unacceptable is national party leaders and senior operatives, whose influence is diminished by the fact that they are loathed by the GOP base (a dynamic that helped give rise to Trump in the first place).

One serious knock on Trump is that his unfavorable rating is poorer than those of Cruz and Rubio. A Quinnipiac poll found his rating with Republicans at 62 percent favorable, 31 unfavorable. Cruz’s were 62 percent favorable, 23 percent favorable; Rubio’s were 64 percent favorable, 17 percent unfavorable.

Sullivan, Rubio's campaign manager, wrote in his memo that the Floridian "has the highest favorability of anyone in the race" and has "the most room to grow."

But for now, at least, there is scant evidence to suggest Trump’s unfavorable ratings are at a critical mass to prevent him for getting the nomination.

4. 'Second choice' votes aren't all anti-Trump

While a crowded field arguably helps Trump more than a small field, a NBC/SurveyMonkey poll released Thursday indicates that supporters of other candidates would not unify against Trump as others drop out.

The survey found that Bush backers are torn between Rubio (19 percent), John Kasich (16 percent), Cruz (12 percent) and Trump (11 percent). Kasich fans are torn between Rubio (24 percent), Trump (16 percent) and Cruz (10 percent). Ben Carson supporters split between Cruz (24 percent), Trump (22 percent) and Rubio (16 percent).

"I think they live in a fantasyland right now," former Republican House Speaker Newt Gingrich said Monday on Fox News, referring to the GOP establishment. If you take Trump, Cruz, and Carson's votes, "they have been consistently above 60 percent everywhere in the country, if you pool together all of the insurgents."

As for Trump's two chief rivals, it is similarly unlikely that if one drops out, the other would gain all of his voters. The NBC/SurveyMonkey poll found that Rubio supporters prefer Cruz over Trump as their second choice by a margin of 31 to 17 percent. Cruz supporters split 33 percent for Rubio and 26 percent for Trump.

Taken together, the data suggest that defeating Trump will require weakening his support among Republicans, rather than simply turning the primary into a one-on-one contest with the front-runner.

Before it's here, it's on the Bloomberg Terminal. LEARN MORE