Republican presidential candidate Marco Rubio outlined the stakes of the 2016 election for an audience in Reno, Nev., saying he would be "a president for all Americans." (Reuters)

Republican presidential candidate Marco Rubio outlined the stakes of the 2016 election for an audience in Reno, Nev., saying he would be "a president for all Americans." (Reuters)

Mainstream Republican donors and elected officials flocked to Sen. Marco Rubio (Fla.) on Monday amid a growing sense that he is the last best chance to prevent Donald Trump from running away with the ­Republican presidential nomination.

But Rubio’s path remains narrow and perilous. He has yet to win a state, and a raft of major March 1 contests known as ­“Super Tuesday” offers 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 governor Jeb Bush ended his campaign Saturday.

Fielding questions from reporters here Monday morning, Rubio didn’t predict any imminent victories.

Many mainstream Republicans see Rubio as the strongest alternative to Donald Trump, above. (Alice Li/The Washington Post)

“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, referring to primary contests that begin March 15.

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

[GOP candidates chart different paths ahead of Super Tuesday]

Throughout Monday, a string of former Bush backers from across the country gravitated to the senator from Florida, including former Republican presidential nominee Robert J. Dole and Sen. Orrin G. Hatch (R-Utah). In South Florida, Republican Reps. Ileana Ros-Lehtinen, Mario Diaz-Balart and Carlos Curbelo and former congressman Lincoln ­Diaz-Balart — all of whom had backed Bush — announced their support.

Rubio also picked up backers who previously stood on the sidelines, such as former Minnesota governor Tim Pawlenty and Sens. Jeff Flake (R-Ariz.) and Thom Tillis (R-N.C.).

On the donor side, in addition to Kilberg, former ambassador 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 big donations to support Bush.

Phil Rosen, a New York lawyer who is a major Republican fundraiser, said he has spent the past two days on the phone with former Bush donors who are eager to join the Rubio effort.

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“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.

Rosen said 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 Super Tuesday states, a super PAC supporting Rubio casts Trump as “erratic” and “unreliable.” It says Sen. Ted Cruz (R-Tex.), another top rival, is “calculated” and “underhanded.”

Rubio campaigned in Nevada on Monday in advance of the state’s Tuesday caucuses, which seem to favor Trump but are small and unpredictable. At his campaign stops, Rubio talked up his personal ties to the state, where he lived as a child.

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.

[Fall of the House of Bush: How his name and Donald Trump doomed Jeb]

As the pace picks up, Rubio has adopted a broader message, sounding general-election notes in recent days as he has tried 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, Tenn., 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 GOP’s minority outreach issues and responding with a story about the ethnically diverse group of South Carolina leaders who backed him.

“I said, ‘Well, just this afternoon, I was onstage 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 and Monday by Sen. Dean Heller (R-Nev.), a former Bush backer. Heller told the crowd in North Las Vegas that the race is a “two-man show” between Rubio and Trump and repeated himself in Reno on Monday. He pointedly left out Cruz, who won the Iowa caucuses and who finished close behind Rubio in South Carolina.

At a rally in Minden, which was held outside on a sunny and temperate afternoon, Heller joked, “I heard that Trump kicked El Niño out of the country.”

[Fix: The idea that Trump will derail himself seems like wishful thinking]

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

A 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 leaving the race. Kasich will campaign 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 voting 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 decisive wins in New Hampshire and South Carolina and campaigning hard in Nevada.

At a Sunday rally for Rubio in Little Rock, Seth Flynt, 28 of Sherwood, Ark., held up an “Anyone but Trump” sign.

Flynt embodied the challenge Rubio faces in trimming down the field to a one-on-one showdown with Trump. He explained that Rubio was not his first or even second choice. His pick: retired neurosurgeon Ben Carson, who is still in the race despite poor showings in the early states.

Matea Gold in Washington contributed to this report.