WASHINGTON (Reuters) - U.S. Republicans in Washington are coming to grips with what many of them not long ago considered an unimaginable reality: Donald Trump is likely to be their presidential nominee and standard-bearer. The prospect of Trump winning the Republican primary had been the stuff of Washington jokes, whispered hallway conversations and eye-rolls, even as he led in public opinion polls for months and dominated debate after debate. But with the brash billionaire now winning three straight contests in New Hampshire, South Carolina, and Nevada, denial is giving way to a mostly gloomy acceptance that he may have too much momentum to be stopped, especially if wins big in key Southern primaries next week that look favorable to him. “It fills all of us with concern and dread,” said Senator Jeff Flake of Arizona, who has endorsed fellow Senator Marco Rubio of Florida, considered the main hope of the Republican establishment to derail Trump’s march to the nomination. That march was given a boost on Friday when New Jersey Governor Chris Christie, a Republican presidential candidate who dropped out of the race after a dismal finish in New Hampshire, became the first major establishment Republican to endorse Trump. “There is no better fighter than Donald Trump,” Christie said at a news conference with Trump in Texas. Trump has vowed to scrap U.S. trade deals, slap a tariff on imported goods and raise taxes on hedge-fund managers, as well as retain some sort of mandate to purchase health insurance - clashing with the free-market principles that have long underpinned Republican economic policy.  Some Republicans in Congress, such as Flake and Senator Lindsey Graham of South Carolina, said a Trump nomination would do enormous damage to the party and predicted a heavy election defeat in November to the eventual Democratic nominee. “I am like on the team that bought a ticket on the Titanic after we saw the movie,” said Graham, contending that Trump would be “slaughtered” in the general election.  In a Republican presidential debate in Houston on Thursday night, another Trump rival, Senator Ted Cruz of Texas, challenged him on his electability, citing ties to Democratic front-runner Hillary Clinton such as a donation to the Clinton Foundation. Trump responded by ridiculing Cruz for his inability to win more than the early voting state of Iowa and taunted him for being behind the billionaire in opinion polls in Cruz’s home state of Texas. Said Trump, “If I can’t beat her, you’re really gonna get killed, aren’t you?” Another Rubio supporter, Representative Carlos Curbelo of Florida, told Reuters he would not back Trump if he were the nominee. “If the nominee is a fraud, and someone who’s offensive, and incapable of being an effective president like Donald Trump, I won’t support him,” Curbelo said.  Other Republicans tried to be more optimistic.  “I don’t think his nomination would be catastrophic,” said Senator Susan Collins of Maine. She said she did not believe, as some strategists fear, that having Trump on the ballot in November would hurt Republican chances for holding onto control of the Senate, where the party currently has a 54-46 edge. Conservative economist Arthur Laffer, an adviser to former President Ronald Reagan who has been counseling Trump on tax policy, said he was convinced the real estate mogul was open to sound advice. Laffer recalled Trump telling him: “‘Look, if you’ve got a better idea than I’ve got, tell me, and I’ll change.’” Senator John Thune of South Dakota suggested Trump’s presence could help by bringing more voters to the polls.  “There’s a lot of energy, a lot of intensity on our side,”  Thune said.  Privately, lobbyists, economists, and analysts expressed deep concern about having Trump, who has proposed building a wall along the southern U.S. border and imposing a temporary ban on Muslims entering the country, as the face of the party.  “There are a lot of people who are really freaked out,” said Douglas Holtz-Eakin, who was the chief economic policy advisor to 2008 Republican presidential candidate John McCain. “He seems to be winging it.”  Conservative policy-makers worry that Trump’s pitch to voters is based on his management skills rather than conservative principles.  Juleanna Glover, a prominent Republican communications consultant, told Reuters that Trump’s ascent “spells the death of the party’s sentient and cohesive governing framework.” Two Republican business lobbyists, who also asked to remain unidentified, told Reuters that they are very concerned about Trump, chiefly because they do not know what he stands for.  They said they have no sense of certainty because Trump’s positions on issues such as tax, trade, and regulation range from being only vaguely understood to completely unknown.  By vowing to make America “win” again abroad while going into little detail on his foreign policy plans, Trump is also stirring concern in Washington national security circles.  A high-ranking official at a conservative think-tank, who spoke on condition of anonymity because his job requires him to steer clear of partisan politics, said: “Every serious student of American strategy is sick to their stomach about the possibility of Trump being the Republican nominee.”  Robert Kagan, a conservative foreign relations expert at the Brookings Institution think tank, said in a column for the Washington Post on Thursday that he would vote for Clinton rather than Trump. “The party cannot be saved, but the country still can be,” he wrote. Paul Ryan, the speaker of the U.S. House of Representatives and a leading voice on conservative economic policy, was asked Thursday whether he could work with someone like Trump if he became the nominee.  “We’ll cross these bridges when we get to it,” Ryan said. “But I do believe that we will be able to unify as a party.” Asked about the hand-wringing in the Republican establishment about Trump, his campaign manger, Corey Lewandowski, said, “Look, we’ve got relationships with those guys and we talk to them all the time.  “But I think what you find is that, you know, politics as usual in Washington, D.C., is not something that the American people want,” he said. Lewandowski added that voters “sent a very clear message” in the three early voting states where Trump won nominating contests “that they want someone who is going to make fundamental change.” Asked if Trump’s campaign would work harder to win establishment endorsements as he got closer to the nomination, Lewandowski pointed to former Florida Governor Jeb Bush, an early establishment favorite who quit the race on Saturday. “If endorsements mattered,” he said, “Jeb Bush would be the nominee.”  (Reporting by Susan Cornwell, Richard Cowan, Kevin Drawbaugh, Jason Lange, Arshad Mohammed, David Morgan, James Oliphant,  Matt Spetalnick and Emily Stephenson; Writing by James Oliphant; Editing by Stuart Grudgings and Frances Kerry) This article was funded in part by SAP. It was independently created by the Reuters editorial staff. SAP had no editorial involvement in its creation or production.