WASHINGTON, D.C.—As Donald Trump continued to gain momentum in his bid for the White House, Republican officials, donors, and members of Congress declined on Wednesday to join forces against him.

“We’ve heard this kind of alarmism before,” a senior Republican senator told Slate, responding to warnings that a Trump nomination would doom the Republican Party. “Remember global warming? Gases, temperatures, the sky is falling? It’s all speculation. I’ll believe this Trump wave when I see it.”

With his victory in Nevada on Tuesday—the third consecutive contest he has won by more than 10 percentage points—Mr. Trump has captured three of the first four primaries and caucuses. He now has 81 delegates, more than four times as many as his nearest rival. Polls taken in the last week show Mr. Trump leading handily in nearly every state that will host a primary or caucus in the next month. But party insiders remain skeptical.

In interviews, several GOP leaders and operatives doubted Mr. Trump’s prospects. Some ridiculed a widely shared hockey-stick graph of delegate totals that projects him ascending toward a point of no return. Others questioned the measurements that have supposedly recorded his rise. “There’s a long history of skewed polls,” said a veteran strategist. “Liberals control the polling industry, academia, the media. These are the same people who told us Romney was going to lose.”

Much of the doubt seems to arise from past elections in which dire predictions failed to materialize. “It’s always somebody—Robertson, Buchanan, Huckabee—and it’s always hype,” said a former Republican National Committee official. Another source familiar with deliberations inside the party dismissed Mr. Trump’s surge as a discredited hypothesis. “Remember back in January, when they told us he was hot? Then, boom, he freezes up in Iowa. We’re not going to fall for that again.”

Some Republicans in Congress said the evidence that Mr. Trump had gained ground in the contest for the nomination was not “settled” or “conclusive.” Others questioned the connection between his standing in the polls, the percentage of votes he has won in each state, the number of delegates he has accumulated, and his chances of winning the nomination. “I understand the theory,” said a Republican member of the House Math and Science Committee. “But there’s no direct evidence that all these things are related.”

Sen. Mitch McConnell of Kentucky, the Senate majority leader, said he had not seen such a difficult evidentiary dispute since the controversies over the alleged health effects of coal mining and tobacco. “Look, I’m not a political scientist,” Mr. McConnell observed. “For every so-called expert who says Trump is going to win, I can show you somebody else who says he won’t. Until we know for certain, let’s all take a deep breath.”

At conservative think tanks, a lively debate has been underway about Mr. Trump’s success. Some scholars acknowledge that Mr. Trump is leading the race but express doubt as to whether, or to what extent, Republican support for his candidacy is responsible for this phenomenon. “I’m not denying that Republicans have played a role, but let’s not overstate it,” said Peter Nelson, a Koch Industries Fellow at the Heritage Foundation. “Delegate counts go up, delegate counts go down. Delegate counts have been changing for hundreds of years. Attributing causes seems kind of premature.”

To be sure, there are voices in the party who believe that Mr. Trump is leading the race and that he will win it unless Republican officeholders, donors, and rival candidates mobilize against him. At least one strategist, who declined to be named, has proposed such a campaign in conversations with potential financiers. “I saw his PowerPoint,” said a Wall Street executive who met recently with the strategist. “He had polls and delegate projections and a timetable of what’s left on the primary calendar. I don’t know if he’s a RINO or just trying to make a fast buck. These ‘Trump is winning’ characters have an agenda, and they cook their numbers to con us into paying for it.”

Republicans who have lost faith in the party’s leaders and donors are placing their hopes in Sen. Marco Rubio, who has trailed Mr. Trump in every caucus and primary so far. There was widespread speculation on Monday and Tuesday that Mr. Rubio, who had refrained from criticizing Mr. Trump, might decide to confront him. But Mr. Rubio, speaking with reporters Wednesday, begged off. “Other candidates have been picking up support for as long as the Earth has been around,” Mr. Rubio said at a campaign stop in Florida. “According to the Bible, that’s at least since last year. And there’s no evidence that our campaign, acting alone, can stop that process. I am not going to weaken our campaign and endanger the jobs of thousands of volunteers to pursue unilateral action that would not solve the problem, even if it exists.”

Mr. Trump, reached for comment aboard his private jet outside Clem’s Jerky Shack in Sticksville, Georgia, laughed at Mr. Rubio and others who have chosen to ignore his “yuge” lead. “What do they think I am, a hoax?” Mr. Trump asked. “I’m going to win this thing. Believe me.”