At Fox Business Network’s debate in North Charleston, S.C., candidates vied for momentum ahead of the Iowa Caucus. Washington Post reporter Robert Costa names the winners and losers from that night. (Dalton Bennett/The Washington Post)

As the presidential primary race moves into a more urgent and combative phase, there is growing acceptance among Republicans, including the Washington and financial elite, that Donald Trump and Ted Cruz are the two candidates most likely to become the party’s nominee.

Their commanding performances at the sixth debate — along with their continued dominance in national and early state polls — have solidified the conclusion of many Republicans that the campaign is becoming a two-person contest.

Long expected to become a race between an outsider and an establishment candidate, it is coming down instead as one between two outsiders, with dwindling time for their rivals to change the trajectory before the Iowa caucuses on Feb. 1.

There is hope that one of the four establishment candidates may emerge as a consensus choice and consolidate support. The two who seem best positioned to do so, Sen. Marco Rubio of Florida and New Jersey Gov. Chris Christie, had some sparkling moments in Thursday’s Fox Business Channel debate but left the stage bruised and squabbling. They returned to the campaign trail on Friday aiming more firepower at each other than Trump or Cruz.

“Trump and Cruz sucked all the oxygen out of the room, which is bad news for the establishment folks,” said Barry Bennett, a veteran GOP strategist who recently resigned as campaign manager to Ben Carson. “It doesn’t look like much is going to stop them. They’re in a tier off to themselves, and I think our nominee is going to be Cruz or Trump.”

Speaking in the spin room after the Fox Business GOP debate on Jan. 14, Donald Trump continued his attacks against Ted Cruz. (Dalton Bennett/The Washington Post)

Republican donors, who had long assumed that the outsider candidates would self-destruct and that voters would rally around someone such as former Florida governor Jeb Bush, are suddenly adjusting their thinking and strategies.

Spencer Zwick, the national finance chairman for Mitt Romney’s 2012 campaign, said power brokers and financiers are now trying to cozy up to Trump in various ways, such as reaching out through mutual friends in New York’s business community.

“A lot of donors are trying to figure their way into Trump’s orbit. There is a growing feeling among many that he may be the guy, so people are certainly seeing if they can find a home over there,” he said.

Zwick cautioned that he does not necessarily predict that Trump will win, only that he could, and said some of these donors are talking regularly with other candidates as well.

“I’ve never subscribed to the view that [Trump] can’t be the nominee, that ‘Oh, don’t worry’ take on things,” he said. “It’s naive and foolish to think he can’t be the nominee.”

In another sign of acceptance of the front-runner, Brett O’Donnell, a longtime debate coach for GOP presidential candidates, said Trump’s performance skills have improved notably and have enhanced his reputation among the political class.

“He is starting to get the art of political debate,” O’Donnell said. “He’s starting to understand how to drive a message, how to seize the competitive advantage over his opponents. When he first started out, he would just say awful things and mix it up. Trump has now figured out how to sound more reasonable.”

Republican presidential candidate Texas Sen. Ted Cruz greets guests after speaking at a Conservative Leadership Project event on Friday in Columbia, S.C. (Scott Olson/Getty Images)

Trump says he is rejecting large campaign donations, though he has relied heavily on small-dollar donations as well as his own money to fund his campaign.

Cruz, who has a more traditional finance operation, is aggressively courting establishment financiers. He held a fundraiser Monday at the New Orleans home of Mary Matalin, a Bush family loyalist and close adviser to former vice president Richard B. Cheney, and drew support there from some traditional party players.

Heading into the Iowa caucuses, the fault lines are drawn.

“This has become a two-person race: Cruz and Trump,” said Bob VanderPlaats, a prominent Iowa evangelical leader who is backing Cruz. “There were almost tears on the stage. You could sense it, feel it and see it.”

The debate brought an abrupt end to the chumminess that long existed between Trump and Cruz. The billionaire mogul and the Texas senator, who have thrown political correctness to the wind, were at loggerheads Friday as they sought to undermine each other.

Their debate-night argument over whether the Canadian-born Cruz meets the constitutional requirements to serve as president spilled onto the campaign trail Friday in Iowa, where Trump revived his so-called “birther” attack in a CNN interview. He also hit Cruz over his recent admission that he had failed to properly disclose a loan during his 2012 Senate campaign.

Meanwhile, Cruz is starting to make an assertive case that Trump is an untrustworthy and unprincipled conservative. At the debate, the senator raised Trump’s past claim that he had “New York values,” charging that his once-liberal views on social issues were out of step with red America.

Trump got the better of Cruz by calmly and emotionally recalling New York City’s painful recovery from the Sept. 11, 2001, terrorist attacks.

But Cruz advisers believe the “New York values” attack will have lasting currency among conservatives in Iowa especially. Through social media and releases to reporters, the Cruz campaign pushed out archival footage of Trump in a 1999 interview on NBC’s “Meet the Press” with the late moderator Tim Russert saying that his “New York values” were different than “Iowa values.”

Cruz’s team professed confidence on Friday, eager to dismiss Rubio and the other establishment candidates as afterthoughts and vowing that their boss was ready for a one-on-one fight with Trump for the nomination.

“It’s a two-person race,” Cruz spokesman Rick Tyler said. “There is no one in the moderate lane who seems to be emerging. The party doesn’t seem to want to elect a moderate. Our argument is, there is only one conservative who has a path.”

Some outside strategists are less bullish on Cruz, however. Eric Fehrnstrom, a former Romney adviser, said Trump remains in “total command of the field.” He said Cruz has yet to put the issue of his constitutional eligibility to rest and suggested he “produce his own expert,” such as retired Supreme Court justice Sandra Day O’Connor, to issue a statement confirming Cruz’s qualifications.

“As long as there are differing but inconclusive interpretations of the law, it will remain unresolved,” Fehrnstrom said.

Bush also remains a factor, pushing hard against Trump in particular during Thursday’s debate and then landing the endorsement on Friday of Sen. Lindsey O. Graham (R-S.C.), who recently dropped out as a 2016 contender.

For Rubio, his sights are set on overtaking Cruz — though his history pushing immigration reform in 2013, which Cruz and his allies have been highlighting, stands as an obstacle.

Campaigning in New Hampshire on Friday, Rubio upped his tempo and darkened his pitch, voicing anger that echoed Trump. Rubio charged as he did in the debate that the Texas senator supports a European-style value-added tax, or VAT, referring to Cruz’s proposal to replace corporate income and payroll taxes with a “business flat tax” of 16 percent. Cruz disputes the VAT comparison.

At a Cruz event in Columbia, S.C., a man delivered Rubio campaign fliers to reporters showing Cruz, against a backdrop of the European Union’s flag, as a sap for a new tax on all goods. The man dropped the folded document like napalm and hot-footed it out the door.

In New Hampshire, where the establishment candidates are deadlocked in the polls, Rubio mostly fired away at Christie, continuing a fight that broke out between them at the debate.

Christie denied Thursday that he ever supported President Obama’s nomination of Supreme Court Justice Sonia Sotomayor. On Friday, Rubio produced a piece of paper to show otherwise.

“I have his quote right here,” Rubio said eagerly. Reading from it, he said, “I support her appointment to the Supreme Court and urge the Senate to keep politics out of the process and confirm her nomination.”

Outside Rubio’s town-hall event, a Christie supporter distributed copies of a 2013 fundraising email Rubio sent on Christie’s behalf commending the governor for his reforms in New Jersey.

Ray Washburne, Christie’s national finance chairman, predicted the race would be monopolized by Trump and Cruz until a consensus choice emerges from the field of four establishment candidates, which includes Ohio Gov. John Kasich.

“We’re going down a highway with Cruz driving down one lane, Trump on the other, and the other four — Christie, Jeb, Marco and Kasich — we’ve got a traffic jam,” Washburne said. “Right now it’s ‘Let’s wait and see.’ ”

Sean Sullivan in Claremont, N.H., Jose A. DelReal in Des Moines and David Weigel in Columbia, S.C., contributed to this report.