Republican presidential candidates sparred over experience, took jabs at their Democratic opponents and even shared a few laughs during CNBC's main debate in Boulder, Colo., on Oct. 28. (Sarah Parnass/The Washington Post)

The fight for the 2016 Republican presidential nomination appears to be moving into a new, more fluid phase.

No longer is the question merely whether or how Donald Trump can be stopped.

The recent rise in the polls of retired neurosurgeon Ben Carson — Trump’s low-key stylistic opposite — has shown that the celebrity billionaire may not be the only one who can tap the appetite of many in the party’s angry base for an outsider.

And after Wednesday’s chaotic and freewheeling debate, there also is a new dynamic on the establishment side of the race.

Former Florida governor Jeb Bush’s once-formidable campaign appears to be nearing a state of collapse, made worse by his flailing on the stage in Colorado.

The Fix's Chris Cillizza explains how Republican presidential candidate Jeb Bush missed his opportunity and opponent Marco Rubio had a strong performance in the third GOP debate. (Zoeann Murphy/The Washington Post)

That has provided an opening to his onetime ally, Sen. Marco Rubio of Florida, who is getting a new look from the party establishment — an ironic situation, given Rubio’s roots as an insurgent tea party favorite in 2010.

“Marco Rubio now has probably the best shot to emerge as the mainstream alternative to Trump and Carson,” said Ari Fleischer, who was press secretary for President George W. Bush.

More broadly, Fleischer, who is not committed to any of the 2016 candidates, predicted that the GOP is about to enter “a condensed version of where it was four years ago, where the party is volatile and shopping around.”

That could help Ted Cruz, who also made a strong showing in the debate. The firebrand senator from Texas, widely despised by the Washington Republican hierarchy, is looking to nudge out Trump and Carson among voters who are looking for a candidate to supplant the old order.

“I don’t think the party is going to nominate anybody who has not been elected before,” said Stuart Stevens, who was a top strategist for 2012 GOP nominee Mitt Romney.

Also likely to force some clarity in the coming weeks is the calendar. The first contest, in Iowa, is barely more than three months away.

So the focus for all the leading contenders will have to shift, from raising their profiles nationally to refining the strategies and organization it will take to put specific states in their column.

“The campaign is really in a nuts-and-bolts stage,” said Trump’s campaign manager, Corey Lewandowski. “It’s about getting on the ballot, organizing, and making sure people understand what caucuses are in Iowa and other states and teach them how to participate. It’s about getting people committed and ready.”

Lewandowski said Trump’s campaign is already hiring staff for states that do not hold their primaries until March.

“Our approach is to execute and meet the criteria to get on the ballot in all 50 states, in five territories and in the District of Columbia. That’s a full-time job,” he added. “This campaign is doing all of the things necessary for long-term success.”

At the same time, Trump seemed to be moderating and refining his message as he campaigned outside Reno, Nev., Thursday afternoon.

Where he previously has devoted his rallies to slinging insults at his opponents and boasting about his poll numbers, Trump focused instead on describing the professional and life experience he would bring to the White House.

He also cited issues where he claimed to have led before it was a popular thing to do — including opposing the Iraq war and aggressively combating illegal immigration.

“That’s the kind of thinking we need in the country,” Trump said. “A lot of the people in the audience, maybe in your small way you have that same thinking.”

Bush, campaigning in New Hampshire, insisted that his struggling candidacy should not be counted out. The former Florida governor, who is polling in the single digits almost everywhere, insisted, “We’re doing fine.”

But he appeared to acknowledge that he had not helped his prospects with his showing in the debate.

“Look, there are two types of politicians. There are the talkers and there are the doers,” Bush told the crowd. “I wish I could talk as well as some of the people on the stage, the big personalities on the stage, but I’m a doer.”

Rubio, meanwhile, must capi­tal­ize — quickly — on whatever interest and momentum may be generated by his debate performance. He spent the morning making the rounds of six network and cable television shows and the remainder of the day fundraising in Denver and Chicago. He will be in Iowa on Friday.

On “CBS This Morning,” Rubio declined to criticize Bush personally and said the differences between the two will be fleshed out in terms of policy.

“I’m going to continue to tell people who I am, what I’m for. There are policy differences between us — we’ll discuss those. Americans deserve to hear those. But I’m not going to change my campaign,” he said.

“Jeb is my friend, I have admiration for him. I’m not running against him. I’m running for president.”

Cruz, meanwhile, is building what GOP insiders say is a strong organization. The campaign says that it has 77,000 volunteers on the ground, with 6,000 in the first four voting states.

Fundraising has also been robust — and was reignited by the debate, during which the senator trained his fire on the CNBC moderators and the media. A Cruz aide said more than $1 million had poured in since Wednesday night’s debate.

There will be growing pressure on candidates who are getting no traction to get out of the race. On Thursday, for instance, the New York Times editorial board called upon New Jersey Gov. Chris Christie to abandon his bid.

“New Jersey is in trouble, and the governor is off pursuing a presidential run that’s turned out to be nothing more than a vanity project,” the paper wrote. “Mr. Christie’s numbers are in the basement, and he’s nearly out of campaign cash. This is his moment, all right: to go home and use the rest of his term to clean out the barn, as Speaker John Boehner would say.”

Christie, for his part, tweeted, “Can’t read the article because I don’t have a subscription, but I can tell you this — I am not going anywhere.”

Robert Costa in Colorado and Jenna Johnson in Nevada contributed to this report.