LAS VEGAS — A wave of mainstream Republican donors and elected officials are swiftly flocking to Sen. Marco Rubio, R-Fla., amid a growing sense that is he is the last best chance to prevent Donald Trump from running away with the Republican nomination.

But Rubio’s path remains narrow and perilous. He has yet to win a state, and a raft of major contests on so-called Super Tuesday, March 1, offer few obvious chances for him to do so. And if Trump keeps racking up wins, it will become more difficult to blunt his progress.

Increasingly, there is a recognition among Republican elites that if Trump is not slowed by the middle of March, it may be too late to prevent him from winning the nomination.

“The window is closing and we need to move now,” said Bobbie Kilberg, a major Republican donor who lined up behind Rubio after former Florida Gov. Jeb Bush ended his campaign on Saturday.

Fielding questions from reporters here Monday morning, Rubio wasn’t predicting any imminent victories.

“We look forward to continuing to add delegates to our count and as we get into the winner-take-all states I think we’re going to be in a very strong position,” he said.

Bush’s departure from the race is arming Rubio with a much-needed injection of establishment money and structural support. Those who sided with Bush or were reluctant to cross him are now free to back Rubio.

On Monday, former Minnesota Gov. Tim Pawlenty and Sens. Jeff Flake, R-Ariz., and Thom Tillis, R-N.C., endorsed Rubio. In Miami, Republican Reps. Ileana Ros-Lehtinen, Mario Diaz-Balart and Carlos Curbelo and former Rep. Lincoln Diaz-Balart – of whom had backed Bush – also announced their support.

On the donor side, in addition to Kilberg, Amb. Francis Rooney, who gave more than $2 million to a pro-Bush super PAC through his holding company, is now with Rubio. So is financial industry executive Muneer Satter, who also made a big investment on behalf of Bush.

New York attorney Phil Rosen, a major Republican fundraiser, said he has spent the last two days on the phone with former Bush donors now eager to join the Rubio effort.

“They have a lot of disappointment about Jeb, but they are ready to put full steam ahead for Marco,” said Rosen, who said he has gotten commitments from 15 top Bush bundlers.

“I am going to continue to reach out to literally every person that was on the Bush campaign,” he said, adding that he has not encountered any residual bitterness from the campaign clashes between the two men.

In a new ad released Monday that will run in March 1 states, a super PAC supporting Rubio casts Trump as “erratic” and “unreliable.” It says Sen. Ted Cruz, R-Texas, another top rival, is “calculated” and “underhanded.”

Rubio is campaigning here in Nevada on Monday in advance of the state’s caucuses on Tuesday, which seem to favor Trump but are small and unpredictable. Rubio has been talking up his personal ties to the state, which he lived in for a period as a child.

A week later on March 1, Rubio’s most pressing goal will be to eclipse the threshold required – as high as 20 percent of the vote in some states – to qualify for delegates in the states holding contests that day, most of which are seen as friendlier to Trump or Cruz.

Beyond that, Rubio is looking to the delegate-rich states of Florida and Ohio on March 15, which will award delegates on a winner-take-all basis. Rubio’s backers concede that a loss in his home state to Trump would likely be a fatal blow.

As the pace picks up, the Florida senator has adopted a broader message, sounding general-election notes in recent days as he tries to bolster his central argument: that he is the most electable candidate left in the GOP field.

“Americans are the descendants of people that came here whether it was two centuries ago or two years ago because they refused to live in a society that told them that they could not be who they wanted to be,” Rubio said in Franklin, Tennessee, on Sunday before his largest crowd of the campaign. “America is the descendants of slaves who overcame that horrifying institution to claim their stake to the American dream.”

In a North Las Vegas hotel ballroom Sunday night, Rubio recalled recently being asked about the Republican Party’s minority outreach and responding with a story about the ethnically diverse coalition of South Carolina leaders who backed him.

“I said, well, just this afternoon, I was on stage receiving the endorsement of an Indian-American governor from South Carolina, who has endorsed a Cuban-American from Florida. And I was standing next to the African-American Republican senator from South Carolina. That sounds pretty minority to me,” he said.

Rubio was introduced Sunday night by Sen. Dean Heller, R-Nev., a former Bush-backer. Heller told the crowd at a hotel casino here that the race is a “two-man show” featuring Rubio and Trump. He pointedly left out Cruz, who won the Iowa caucuses and who finished close behind Rubio in South Carolina.

Rubio will campaign in Minnesota and Michigan on Tuesday, which vote on March 1 and March 8, respectively. There, he will continue his strategy of focusing on major metropolitan areas and suburbs.

Another looming threat to Rubio, particularly in the Midwest, is Ohio Gov. John Kasich, a centrist who finished second in New Hampshire and is signaling that he has no intention of stepping aside. Kasich is also campaigning in Virginia, Georgia, Mississippi and Louisiana this week.

In the South, Cruz – who was bruised by his third-place showing in South Carolina – remains a major obstacle to Rubio. The Texan has staked his campaign heavily on a collection of Southern states on March 1.

And then there is Trump, who is ahead in polling and seemingly poised to compete everywhere. Rubio aides are confident that Trump has a lower ceiling of support than their candidate. But the front-runner is fresh off two decisive wins in New Hampshire and South Carolina and will campaign hard here in Nevada.

At a Sunday Rubio rally in Little Rock, Seth Flynt, 28 of Sherwood, Arkansas, held up an “Anyone But Trump” sign and embodied the challenge Rubio faces in trimming down the field to a one-on-one showdown with Trump.

Flynt explained that Rubio was not his first or even second choice. His pick: retired neurosurgeon Ben Carson, who is still in the race despite having no success in the early states.

Matea Gold in Washington contributed to this report.