Donald Trump cruised to victory in Nevada, building a broad coalition that left his two rivals trailing far behind heading into Super Tuesday. The Washington Post’s David Weigel talks about what this means for Trump ’s chances of securing the GOP nomination. (Alice Li/The Washington Post)

Donald Trump has taken firm control of the race for the Republican presidential nomination with his third straight victory, in Nevada. To deny him that role, strategists say, his leading rivals must quickly change the trajectory of the race and then dig in for what could be a long battle that could go all the way to the GOP convention in Cleveland.

Strategists who have been through past nomination battles say that Sen. Marco Rubio (Fla.), Sen. Ted Cruz (Tex.) and Ohio Gov. John Kasich collectively have until March 15 to turn the race away from the New York billionaire. Each has a must-win test looming in his home state between now and then. But those victories alone might not be sufficient to block Trump’s path.

As for Trump, the front-runner hopes to suffocate his three main rivals in next week’s Super Tuesday contests, particularly in primaries across the Deep South, and to knock them off with wins in each of their home states. In his Las Vegas victory speech after the Nevada caucuses Tuesday night, Trump predicted he would secure the nomination in less than two months and taunted his top three opponents by trumpeting his high poll numbers in Florida, Ohio and Texas.

The first opportunity to alter the campaign could come Thursday night, when the Republican rivals meet here in Houston for their first debate since the departure of former Florida governor Jeb Bush, who quit the campaign Saturday after finishing fourth in South Carolina.

Changing the dynamic, however, depends on whether any of them has a strategy to put Trump on the defensive. So far, Trump’s rivals have focused more on one another than on him, in hopes of becoming Trump’s last viable opponent. Many Republicans fear that strategy will only allow Trump to put a stranglehold on the nomination with continued victories in the coming weeks.

Charts: The delegate math shows Trump may be unstoppable

“The reality is that, until the field starts to narrow, it’s going to be very, very hard to take him out,” said Katie Packer, deputy campaign manager for Mitt Romney’s 2012 campaign and the leader of an anti-Trump super PAC. “I think people need to step up and start taking on Trump. Front-runners don’t just stumble. People trip them.”

After losing the Iowa caucuses to Cruz on Feb. 1, Trump has piled up victories in New Hampshire, South Carolina and Nevada. He heads into next week’s Super Tuesday contests in 11 states with the anti-Trump vote still splintered among his rivals.

Given Trump’s string of successes, his rivals must demonstrate that they can beat him, not just finish ahead of each other. “You have to win,” said Stuart Stevens, chief strategist for Romney in 2012. “Winning transforms a candidate, and losing transforms a candidate. You’re not the same person after you’ve won a bunch of races, and you’re not the same after you’ve lost a bunch of races.”

Winning states does more than change perceptions. It’s the key to amassing delegates. From here forward, the campaign shifts from a battle for momentum to the trench warfare of gathering delegates.

Trump has taken hold of the delegate race in large part by winning all 50 that were at stake in South Carolina last Saturday. The longer the field remains divided, the better Trump’s opportunities to maintain or enlarge that lead.

David Wasserman of the Cook Political Report updated his delegate charts Wednesday morning and concluded that Trump is on pace or slightly ahead of what he would need to win the 1,237 delegates needed to secure the nomination. Rubio and Cruz are behind their pace to achieve that.

Only about 5 percent of the total delegates have been awarded so far. But over the next three weeks, they will come in bunches, with 595 at stake Tuesday and 368 more through March 12.

At that point, the rules shift in what could be a game-changing fashion. Between now and March 14, delegates will be awarded proportionally. Starting March 15, states will be allowed to award delegates on a winner-take-all basis. On that day alone, Florida, Illinois, Missouri and Ohio, with a combined 292 delegates allocated in that way, are among the states or territories with contests.

“It’s important to take a deep breath here,” Rubio said Wednesday on “CBS This Morning.” “It’s not based on how many states you win — it’s based on how many delegates you picked up.” Looking ahead to the March calendar, Rubio said there were “plenty of states out there that — in the winner-take-all category — that if you win them, you more than catch up.”

Over time, Rubio intends to more aggressively draw a contrast with Trump and call him out for inaccuracies or shallow policies. “We believe a majority of Republicans don’t want Trump to be the nominee and will vote for an alternative,” said Rubio communications director Alex Conant. “As this becomes a two-person race, Marco will beat Trump head to head.”

Rep. Peter T. King (R-N.Y.), a Rubio supporter, said Rubio needs to limit Trump’s victories and ensure that Cruz, who came in third in South Carolina, is halted.

“He has to run the hardest campaign he can and stop Trump from sweeping next week,” King said. “He has to make it as clear as possible that he’s now the only option, that Cruz is finished and hit a high-water mark in South Carolina. . . . Cruz had his chance and missed it.”

Jeff Roe, Cruz’s campaign manager, offered a counter view in a Wednesday phone interview. He argued that his candidate still has the best chance to defeat Trump, despite a loss in South Carolina.

“He’s won three in a row,” Roe said of Trump. “That’s not lost on me.” Still, he said that based on the campaign’s projections of states and demographics and possible support, “the road map through March 15th will not be smooth sailing for Donald. For those that are hysterically declaring the race over should take a breath and look at the remaining 46 states.”

At this point, however, none of Trump’s leading rivals sees any incentive to get out of the race. All can sketch a path to the nomination, however difficult, and until they are proven wrong, they will soldier on.

Mike DuHaime, who was chief strategist for the presidential campaign of New Jersey Gov. Chris Christie, said he could imagine long-odds scenarios for Cruz, Rubio or Kasich and said the psychology of believing there is a path to victory, no matter what outsiders think, will keep the candidates going.

“I have been in those rooms,” DuHaime said. “As long as you believe there is a path and a chance, you keep going.”

A fifth Republican, retired neurosurgeon Ben Carson, also remains in the race but is not expected to be a major factor in the March contests.

Meanwhile, Trump’s insular campaign is quickly expanding into a national organization. Staffers are on the ground in states with Super Tuesday primaries and are building grass-roots networks there and in other states such as Florida. Campaign manager Corey Lewandowski said Trump has lined up a “crazy travel schedule” over the next couple of weeks and brushed off any suggestion that his boss was stoppable.

“We have a very strong candidate who is going to be competing in every state,” Lewandowski said. “I don’t expect anything. Our goal is to play everywhere.”

But Republican strategist Phil Cox said the home-state tests are crucial for all three of Trump’s leading rivals. “If you are a candidate other than Trump, from the standpoint of both the national narrative and the actual number of delegates you need to win the nomination, anything less than a win in your home state is a fatal,” he said.

John Weaver, Kasich’s chief strategist, agreed: “There’s no legitimate rationale. If you can’t win your home state in a primary, you can’t expect to move forward.”

The Texas primary comes first, on March 1 along with 10 other Super Tuesday primaries or caucuses. Polls show Cruz with a lead there, although Trump is uncomfortably close behind.

Cruz and his wife, Heidi, are pulling out all the stops to win and on Wednesday picked up the endorsement of Gov. Greg Abbott (R). Cruz’s advisers and allies are confident he will carry the state and are hopeful he gets more than 50 percent of the vote, which would mean he wins all 155 delegates.

Rubio is racing to absorb Bush’s finance and political network in Florida and is expected to pick up endorsements from most officeholders. He is opening at least three campaign offices in Florida and on March 11, the day after a Miami debate, Rubio plans to crisscross the state holding fundraising events with former Bush supporters.

“Some folks are trying to decompress from having backed Jeb and Jeb suspending his campaign, but people are coming on board and recognize that Marco’s the candidate who can win this, who can unite the party and come out the last man standing,” said Rubio spokesman Alex Burgos.

One factor potentially working in Rubio’s favor: Florida is a “closed primary,” meaning only registered Republicans can vote. Trump’s early victories have been fueled in part by independent voters crossing over to support him.

Rubio supporters say victory in Florida could turn the race into a two-person battle. “Marco can win if the non-Trump field narrows quickly and he then seizes the opportunity to fully contrast with ‘The Donald,’ ” Tim Pawlenty, the former governor of Minnesota, wrote in an email.

In Ohio, Kasich, as the sitting governor, enjoys the support of the state Republican Party as well as most GOP officeholders. He also has a relatively high job-approval rating, which his team believes gives him an edge over Trump or any other challenger.

“We’ll do what we have to do to make sure that we do win,” Weaver said. “We’re very confident of it, but we’re not overconfident.”

Rucker reported from Las Vegas. Robert Costa in Washington and Jenna Johnson in Virginia Beach contributed to this report.