Former Florida governor Jeb Bush, right, watches as Donald Trump speaks during the CNN Republican presidential debate at the Ronald Reagan Presidential Library in Simi Valley, Calif. (Mark J. Terrill/AP)

For many of the Republican presidential candidates who expected to be at or near the top of the polls now, finding a solid base of support has proven to be a daunting challenge.

From candidates such as former Florida governor Jeb Bush to Wisconsin Gov. Scott Walker, New Jersey Gov. Chris Christie and Ohio Gov. John Kasich — all members of the governing establishment in the GOP and each twice elected statewide — the summertime success of Donald Trump has created unexpected obstacles.

Instead of moving forward, some of them have been going backward. That has left the GOP no closer to knowing who its 2016 nominee will be than it was when campaigning started many months ago. Instead the party is beset with questions.

Trump, who did not have a strong performance at the second Republican debate Wednesday in Simi Valley, has an obvious vulnerability. He must convince more voters that someone who is such a flamboyant entertainer and enamored with himself can serve effectively as president.

The candidates he has overshadowed remain hopeful that he will never pass that test with enough voters to make him the GOP nominee. But who among them is capable of winning enough hearts and minds to emerge as the eventual winner?

Presidential debates rarely prove to be game changers. Wednesday’s forum was no exception. There was a clear winner: Carly Fiorina, the former Hewlett-Packard chief executive who seized her first opportunity on the big stage to shine. Having gotten the acclaim, her challenge will be to translate positive reviews into real support.

None of the other candidates earned the same kind of plaudits, but they felt like winners nonetheless, largely because they could say they saw signs of weakness in Trump’s candidacy. At different points in the three-hour debate, he looked deflated, not in command on issues and hardly the center of attention. His rivals promise to try to make the days ahead as rough or rougher.

But if his rivals hope he has hit a ceiling and or even begin to sag, they should not expect it to happen overnight. Their early assessments of his staying power have been well off the mark. Trump has proven more resilient as a candidate than they expected. His positions on immigration give him a seemingly solid base among a portion of the Republican Party. His insults of others — on display again in Simi Valley — have not cost him support. His command of reality TV politics far exceeds that of the others.

The outsiders — Trump, Ben Carson and Fiorina — are flourishing, and it’s possible a number of candidates from the various factions of the party will win early contests or accumulate enough delegates to make a muddle of the race well into March.

If the field eventually narrows to a pair of finalists, the contest is likely to pit an insurgent populist such as Trump or someone from the hard-right wing of the party — someone such as Sen. Ted Cruz (Tex.) or former Arkansas governor Mike Huckabee — against a center-right politician with establishment backing.

That wing has traditionally produced the winner. Republicans haven’t nominated a non-politician since Dwight D. Eisenhower. Ronald Reagan was the last pure conservative to beat the establishment favorites, although he was no long shot for the nomination in 1980.

But this is a hard year to speak as an establishment, or center-right, Republican. None of those who would normally be comfortable with that establishment label want to display it. Insiders are posing as outsiders. Sen. Marco Rubio (Fla.) sent out an e-mail Wednesday decrying a broken Washington establishment and saying that’s why he’s running for the presidency rather than for reelection to the Senate.

The competition among those candidates is now a jumble of dashed hopes, unfulfilled potential and great expectations — hardly what was expected as the campaign began to take shape a year ago. That starts with Bush but hardly ends there. Questions abound for others too — among them Christie, Rubio and Kasich.

Bush’s advisers wore some of the biggest smiles in the spin room on Wednesday night. They were elated with his performance, arguing that he had shown energy in contrast to Trump’s contention that he’s a low-energy person and that he had shown fight in confronting Trump repeatedly.

Bush has a super PAC stocked with cash and has begun to spend freely on television ads in Iowa, New Hampshire and South Carolina; at this point $25 million between now and the end of the year and likely to grow. That alone makes him someone the other candidates still see as a potential long-distance runner.

Bush advisers say that as the ads begin to tell the candidate’s story in Florida, his support will rise. But others still question whether he can persuade rank-and-file Republicans to overcome their doubts and questions about him, and they were not convinced that he showed enough in the debate to do so.

Post-debate reviews for Rubio were, once again, positive. Those reviews didn’t help much after the first debate. Maybe over time and with more such performances, that will change. Maybe he will begin to catch fire at the turn of the year. So far, he’s left people impressed with his potential.

Christie advisers also were smiling, given the preponderance of positive reviews. He spoke out forcefully and closed on a high note. But strength of personality has never been his problem. It’s the accumulated baggage from a scandal that never touched him directly, from the economic performance of his state compared with many others and from the scars of being a Republican governor battling a Democratic legislature.

Kasich has built support in New Hampshire, in part with the help of $5 million in television ads aired by his super PAC. He embodies the center-right philosophy that could be effective in a general election — conservative on fiscal issues, compassionate on social safety net policies. He has used both debates to stress experience in Washington and Columbus.

Walker once seemed ideally positioned to be a crosscutting candidate: a two-term governor who had survived tough fights at home and who could appeal to both the establishment and tea party wings of the party. He went into the debate on the defensive and emerged the same way, facing questions about whether he was about to shake up his campaign. No one has seen his stock fall faster than the Wisconsin governor, and no one needs to demonstrate ability as a turnaround artist more urgently than he.

Strategic mistakes — he allowed himself to lean too far to the right — and candidate missteps have left him in a precarious position. His immediate strategy is to add time and resources in Iowa to rebuild support in a state considered critical to his hopes.

The candidates from the establishment wing are obviously watching Trump and wondering how much staying power he has. Obviously it’s more than they first estimated. But they are eyeing one another as much as they are worrying about Trump. Ultimately, one of them must emerge in a dominant position. That’s the other dynamic of this fascinating campaign.