GOP leaders have yet to abandon Donald Trump, but the presumptive Republican presidential nominee said this week he’s ready to kick them to the curb and go it alone against Hillary Clinton if they don’t man up.

In some of his most dismissive comments yet, Mr. Trump said other Republicans need to either get behind him or “just be quiet” — adding to the increasing heartburn among Republicans, particularly those on Capitol Hill whose electoral fortunes are tied to the erratic billionaire.

“You can’t make this up sometimes,” House Speaker Paul D. Ryan said at a press briefing Thursday on Capitol Hill.

Mr. Trump’s admonition came after several weeks of chiding from fellow Republicans who decried his proposal for a temporary ban on Muslims entering the U.S., and who said his attacks on a federal judge’s ethnicity were unbecoming. Mr. Ryan had called it “the textbook definition of a racist comment.”

“The Republicans, honestly, folks, our leaders, our leaders have to get tougher. This is too tough to do it alone. But you know what? I think I’m going to be forced to. I think I’m going to be forced to,” Mr. Trump said on Wednesday.

“Our leaders have to get a lot tougher. And be quiet. Just please be quiet. Don’t talk. Please be quiet. Just be quiet — to the leaders. Because they have to get tougher. They have to get sharper. They have to get smarter. We have to have our Republicans either stick together or let me just do it by myself. I’ll do very well,” Mr. Trump insisted.

GOP observers said Mr. Trump is playing with fire.

“Most people respond better to charm and subtle persuasion than they do to threats, and most victories are won by team efforts versus lone wolves,” said Fred Malek, a White House veteran who serves as finance chairman of the Republican Governors Association. “Trump certainly knows this from his success in business, and why would he think it’s different in politics?”

Mr. Malek said voters want Mr. Trump to “step up and be an inclusive leader.”

But the maverick candidate began his campaign on the other side of the spectrum, accusing Mexico of sending “rapists” and other bad elements of its society to the U.S.

His approach appeared to work in the GOP primary, with his criticism of fellow politicians as “stupid” and “all talk, no action” earning him fans from alienated conservatives and moderates alike.

That has led to an uneasy relationship with the Republican National Committee, as well as GOP lawmakers nationally who’ve had meltdowns over his brash brand of politics, concerned that he is tarnishing the party and its chance of retaining control of Congress in the November election.

This week Maryland Gov. Larry Hogan and Richard Armitage, who served as deputy secretary of state in the George W. Bush administration, announced they were jumping ship and couldn’t support Mr. Trump.

Since wrapping up the GOP nomination in early May, Mr. Trump has shown some signs of willingness to work with his party.

He met with Mr. Ryan and other GOP leaders in Washington in hopes of carving out a common agenda that the party could rally around in the November election.

He also has relied more on teleprompters in order to stay on message after igniting a controversy by asserting that a federal judge of Mexican heritage was out to get him in an ongoing court case involving Trump University.

But the kumbaya moments have been less common than the divisive outbursts.

Republicans on Capitol Hill repeatedly find themselves being asked to defend or distance themselves from the latest pronouncements from their presidential candidate. Some have taken to brushing the questions aside, while others engage — to some extent.

Mr. Ryan, speaking to reporters Thursday, was asked if he would rescind his endorsement of Mr. Trump.

“That’s not my plan,” said the Wisconsin Republican, who was the GOP’s vice presidential nominee in 2012.

Mr. Ryan said too much was at stake to abandon ship.

“We will lose our freedoms in this country, including all of the Bill of Rights, if we don’t robustly defend the separation of powers, and we’re going to fight for those rights on behalf of our citizens so that we remain a self-governing people,” he said.

RNC Chairman Reince Priebus, meanwhile, downplayed the drama by announcing that he was flying with Mr. Trump to Dallas for a rally.

“Reports of discord are pure fiction,” Mr. Priebus said on Twitter. “Great [events] lined up all over Texas. R’s will win in Nov!”

But Mr. Trump’s critics said the New York businessman’s comments show that his post-primary promise to act more presidential has already fallen by the wayside and marks an escalation of a yearlong trend that is eating away at whatever good will Republicans still have for Mr. Trump.

“I think what folks see is that he bites that hand that wants to feed him and then complains that they don’t like being bitten,” said Doug Heye, a veteran GOP strategist who has vowed not to support the billionaire businessman.

“In Trump’s world you either become a Trump acolyte, one of these people defending him on TV that is starting to look like cast of ‘The Addams Family,’ or you’re a heretic,” he said. “The Trump campaign has obviously given up on Trump ever being presidential.”

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