The Republican Party was in a state of pandemonium as independent groups and top GOP figures including Mitt Romney scrambled to throw together a last-ditch effort to deny Donald Trump the presidential nomination, even as some party figures concluded it was now too late to stop the billionaire mogul.

The decentralized and desperate stop-Trump campaign found a possible new leader in Romney, the party’s 2012 presidential nominee, who planned to deliver a forceful, top-to-bottom indictment of Trump on Thursday.

In prepared remarks released in advance of Romney’s speech, Trump is slammed as “a phony, a fraud.”

In a flurry of conference calls and meetings, top Republican donors and strategists laid plans for a multimillion-dollar assault on the front-runner in a series of states holding contests on March 15. Ground zero is Florida, where home-state Sen. Marco Rubio, the leading establishment candidate, is going all in to defeat Trump, who leads in the polls there.

But other Republicans yielded to Trump after he swept seven out of 11 states on Tuesday, the biggest day of balloting yet. Alex Castellanos, a veteran media consultant who earlier in the season had tried unsuccessfully to organize an anti-Trump campaign, said, “A fantasy effort to stop Trump . . . exists only as the denial stage of grief.”

“Trump has earned the nomination,” Castellanos wrote in an email. “Donald Trump whipped the establishment and it is too late for the limp GOP establishment to ask their mommy to step in and rewrite the rules because they were humiliated for their impotence.”

Similarly, William J. Bennett, a Reagan education secretary, said he could not support the anti-Trump movement.

“I’m used to being the moral scold, but Trump is winning fair and square, so why should the nomination be grabbed from him?” asked Bennett, now a conservative radio host. “We’ve been trying to get white working-class people into the party for a long time. Now they’re here in huge numbers because of Trump and we’re going to alienate them? I don’t get it. Too many people are on their high horse.”

[The Take: The GOP has two weeks to take down Donald Trump]

The deepening split in the party came as the field appeared poised to narrow further. Though stopping short of formally suspending his campaign, retired neurosurgeon Ben Carson told supporters Wednesday that he does not see a “path forward.” He scrapped plans to attend Thursday’s Fox News Channel debate in Detroit, a high-stakes opportunity for Rubio, Sen. Ted Cruz (Tex.) and Ohio Gov. John Kasich to take shots at Trump.

The non-Trump candidates hope to prevent him from acquiring the 1,237 delegates needed to secure the nomination. That would push an ultimate decision to the Republican National Convention in July, potentially turning the Cleveland showcase into a hothouse of intrigue, mischief-making and chaos.

The emergence of Romney as a leading Trump antagonist stoked speculation that he might offer himself as a consensus candidate at the convention. But loyalists were adamant that he has no plans to run.

As results showed Donald Trump leading in at least six states on Super Tuesday, Sens. Ted Cruz (R-Tex.) and Marco Rubio (R-Fla.) argued that nominating him would be bad for the Republican party. Here are key moments from their speeches following the March 1 races. (Sarah Parnass/The Washington Post)

“Over time, there’s been a lot of speculation about that,” said former Utah governor Michael Leavitt, a Romney confidant. “He’s heard from many people about that idea, and he continues to be skeptical about the prospect of success.”

Romney’s associates said he is not planning to offer an endorsement when he speaks Thursday at the University of Utah in Salt Lake City, where he has a home.

On Wednesday night, a group of more than 50 conservative foreign policy experts, including former homeland security secretary Michael Chertoff and former deputy secretary of state Robert Zoellick, issued an open letter calling Trump unfit for the office of president.

Even with fewer people in the race, some GOP figures declared that Trump was nearly unstoppable. Eric Fehrnstrom, a former senior adviser to Romney, said it has become almost “impossible for his opponents to catch up to him.”

Mike Murphy, who ran a pro-Jeb Bush super PAC, said the Trump “train may have left the station. I don’t want to be a critic of what’s being tried, but after millions of dollars in ads, it’s more important to narrow the field than to air more ads against him.”

Still, other operatives worked behind the scenes Wednesday on plans for a ruthless ad blitz to discredit Trump by attacking his business career and character. It marks a dramatic escalation of an anti-Trump campaign that until recently had little firepower.

[The GOP’s implosion over Donald Trump’s candidacy has arrived]

The air assault is largely being funded by Conservative Solutions PAC, a super PAC allied with Rubio; Our Principles PAC, a new anti-Trump outfit; American Future Fund, an Iowa-based nonprofit; and the conservative Club for Growth. Some of the groups are working in collaboration.

Conservative Solutions dropped nearly $3 million worth of new anti-Trump ads this week, largely in Florida, Michigan and Illinois, according to Federal Election Commission filings. Our Principles is also launching a new ad in those states, part of a seven-figure buy that will also air on national cable.

“We have a very target-rich environment,” said Katie Packer, who runs Our Principles. “He has left quite a wake of victims in his path.”

Packer said the increased focus on Trump is taking a toll on the front-runner, pointing to Cruz’s victories Tuesday in Texas, Oklahoma and Alaska.

People with knowledge of the group’s activities said a substantial number of new donors have come aboard since January, when Our Principles launched with an initial $3 million donation from Marlene Ricketts, the matriarch of the family that owns the Chicago Cubs. Among the new contributors are billionaire investor Paul Singer, who serves as the Rubio campaign’s national finance chairman.

New contributions are also bolstering the efforts of American Future Fund, a politically active nonprofit that launched a trio of anti-Trump ads online last week centered on the now-defunct Trump University. The organization is spending $1.75 million to put the spots on the air in Florida cities such as Tampa, Orlando, Fort Myers and West Palm Beach. The group also plans to run ads during the next two Republican debates.

“The people who are donating are very concerned not just about what Trump does to the Republican Party, but to conservatives in general,” spokesman Stuart Roy said.

As of Tuesday, super PACs and other independent groups had plowed $16 million into commercials and mailers explicitly going after Trump and an additional $9.4 million into ads that refer to the brash billionaire. In all, that amounts to just 11 percent of the nearly $238 million spent by outside groups on the presidential race, according to FEC filings.

Some of the operatives have been poring over polling data showing that only a small portion of the electorate was aware of negative aspects of Trump’s career.

“Small percentages of those surveyed knew about Trump University, the failure of Trump Mortgage, the KKK controversy,” said Rick Hohlt, a longtime GOP donor involved in the efforts.

That has convinced some activists that they can gain traction against Trump. But even some involved in the new efforts are uncertain whether the coming assault will have an impact.

“You’re going to see a massive ad spend to the tune of tens of millions of dollars,” said one Republican involved in the planning, who spoke on the condition of anonymity to speak candidly. “Fundraising is not a problem. There may be other problems. Trump may be Teflon — this may not stick to him. On March 15, we will have an experiment: What happens when millions and millions of dollars go after Trump?”

[Spray tans, pants-wetting and little hands: The GOP race goes in the gutter]

Some party strategists said none of the attempts to trip up Trump will work unless Rubio, Cruz and Kasich step up their campaigns.

“People sitting around here talking, it’s just a parlor game,” said Charlie Black, a longtime Republican strategist. “There’s nothing you can do behind the scenes. It’s all got to happen out there on the playing field. You’ve got to go beat the guy.”

What Rubio, Cruz and Kasich now are counting on most is depriving Trump of enough delegates that they could force a convention showdown. The prospect of a brokered convention probably overstates what would unfold, in part because there is no sign of the ability of a few power brokers to have their way.

“This is a political marketplace with a set of structured rules,” Leavitt said. “Whoever can get 1,237 delegates will be the nominee. There is a lot of maneuvering within those rules that can occur. But there is no smoke-filled room.”

The convention rules will not be finalized until just before the event opens in Cleveland. About two-thirds or more of the delegates will be bound on the first ballot to back the candidate who won in their state or district. After that, however, they become free agents.

But the possibility of the convention delegates going against the candidate who had amassed the most delegates, even if not a majority, could leave the party even more divided and demoralized heading to the general election.

The convention rules may be moot if the tide keeps lifting Trump. New examples emerged Wednesday of party elites gravitating toward the former reality television star.

Stephen Moore, a conservative economist and former member of the Wall Street Journal editorial board, said he is considering an endorsement. “For me, Trump potentially represents a big expansion of the Republican Party, a way to bring in those blue-collar Reagan Democrats,” Moore said. “That’s necessary if the party is going to win again.”

Scott Reed, who managed Bob Dole’s 1996 campaign, said “the fear is as high as it’s ever been” in the establishment.

“But I’m amazed that people are acting surprised,” he said. “Trump has been building for months, and the voters are speaking.”

Watching Trump talk on Tuesday night about unifying the party, Reed said: “I was struck that he was doing smart things, saying the right things. . . . He has to keep that sort of thing up. Look presidential. Don’t go back into the gutter.”

Dan Balz, Robert Costa, Thomas Gibbons-Neff and Anu Narayanswamy contributed to this report.