If Republicans hope to stop Donald Trump from winning their party’s nomination for the presidency, it may be too late.

Party insiders now regularly characterize Trump’s candidacy as a “phenomenon,” marveling with some disbelief at his resilience and the extent to which he has tapped into voters’ anger. With his dominant performances in the pivotal early primaries, many now concede Trump is the likely Republican nominee, and that a contested convention is the best-case scenario for Trump’s opponents.

Trump is poised to win big in most of the 11 states voting March 1, known as Super Tuesday; from there, little would stand between him and the party convention in Cleveland. Florida Sen. Marco Rubio and Ohio Gov. John Kasich both have predicted they will best Trump in their home states later in March, which could shift the direction of the race, but Trump currently polls ahead of them in both contests.

“I don’t think we can stop him now,” said one Republican elected official. “I really don’t.”

If Trump is the Republican Party’s nominee-in-waiting, however, the party has seemed reluctant to accept it. Elected officials have coalesced in recent days around Rubio, hoping he could take down Trump in a two-person race. But the race is not narrowing to two candidates. Sen. Ted Cruz also insists he would be best positioned to take Trump down. Additionally, aides to Kasich are pressing for a two-person matchup with Trump and have called for Rubio to suspend his campaign.

Republicans who think Trump could yet fizzle are living in a “fantasy land,” former House Speaker Newt Gingrich said on “Fox and Friends” this week. “Donald Trump is tapping into something that’s real.”

And the stars appear to be aligning in his favor.

The surest sign of Trump’s strength might be the growing acceptance of his candidacy among Republican Party stalwarts. This week Trump scooped up his first congressional endorsements. And on Friday, New Jersey Gov. Chris Christie sent shockwaves through the party when he stood beside Trump in Texas to offer his endorsement, saying Trump is “rewriting the playbook of American politics.”

“There is no one who is better prepared to provide America with the strong leadership that it needs both at home and around the world than Donald Trump,” Trump’s former rival for the nomination said.

As former Sen. Bob Dole this week announced his support for Rubio, he noted that a strong Super Tuesday showing by Trump would make it “hard to deny him the nomination.” Donors and corporate interests, meanwhile, are growing hesitant to take Trump on, lest they burn bridges with a future presidential nominee.

“The donor community writ large is beginning to be resigned to Trump and fearful of spending money against him,” said Matt Mackowiak, a Republican consultant based in Texas.

Meanwhile, opinion of Trump among Republican voters is broadening. Peter Brown, assistant director of polling for Quinnipiac University, finds it notable that “the number of voters who say they couldn’t vote for him seems to be dropping a bit.”

“It shows he is also doing something to raise his acceptability,” Brown said.

Another daunting aspect of Trump’s candidacy is his apparent broad appeal across constituencies, according to exit polls from the nominating contests so far. A Trump sweep through Tuesday’s contests, from the evangelical base in the Deep South to the blue-collar regions of Massachusetts and Vermont, will be particularly troubling for his rivals. Cruz has campaigned on consolidating and mobilizing religious conservatives while Rubio has campaigned as the Uniter in Chief, arguing he is the only candidate capable of bringing new and unconventional voters into the fold.

During the debate Thursday night at the University of Houston, Trump argued his candidacy is expanding the GOP. “I’m bringing people — Democrats — over, I’m bringing independents over, and we’re going to build a Republican Party,” he said.

Still, a few weighty challenges remain for Trump. Sam Nunberg, a former adviser to the business mogul who still supports him, believes that Rubio’s candidacy, backed by much of the party establishment and its biggest donors, is “Trump’s biggest hurdle to the nomination.”

“They’re going to make this a protracted fight,” Nunberg said.

Indeed, Rubio’s campaign confirmed this week it is beginning to plan for a contested convention, which would mean no candidate wins the share of delegates needed to lock up the nomination, freeing up the party to choose its nominee on the convention floor.

“We are certainly preparing for it,” said Todd Harris, a top adviser to Rubio, “because I think it’s important for people to understand that this process may go on for a very, very long time.”

Rubio has also ratcheted up his attacks on Trump, beginning in the Republican debate Thursday in Houston. The attacks spilled onto the campaign trail Friday, when Rubio referred to Trump as a “con artist” and joked that Trump might have wet his pants during the debate.

“He’s not going to be the nominee,” Rubio said at an event Friday.

Among other attacks, Rubio has begun to home in on Trump’s campaign message as lacking in important detail. If Trump is to win the nomination and transition successfully into the general election, some strategists suggest Trump will need to begin fleshing out and clarifying his policy stances.

“He wasn’t in politics before, so he’s left a trail that can take you anywhere you want,” said Steve Forbes, the publisher and former presidential candidate. “That’s why he’s got to sharpen it, define it more.”

For Forbes, who ran his presidential campaigns in 1996 and 2000 as a self-funding outsider, Trump’s campaign feels familiar. But Forbes failed to gain traction where Trump has been wildly successful.

“His timing has been impeccable,” Forbes reasoned, pointing to growing anger among voters over the past few years. “Most thought he would be a passing fad, so they didn’t go after him in a concerted way.”

“The party would like to say that Mr. Trump is the (Pat) Buchanan of 1992 or 1996,” said Nunberg, invoking another famous outsider candidate. “I would say he is Reaganesque. But the party still can’t realize that, or is too ignorant of it.”