WASHINGTON — Donald Trump came to the nation’s capital Thursday to unite the Republican Party, but brief verbal scuffles in a meeting with Senate Republicans soured the trip.

Arizona Sen. Jeff Flake told Trump he is uncomfortable endorsing him, the Washington Post reported.

Trump told Flake he’d lose his re-election bid — even though Flake doesn’t face the voters again until 2018.

Trump also went out of his way to criticize two other GOP senators — Ben Sasse of Nebraska, who has staunchly opposed his candidacy, and Mark Kirk of ­Illinois, who took the remarkable step of un-endorsing Trump last month.

Trump’s Senate allies played down the exchanges.

“I think it was a frank discussion,” Sen. Jeff Sessions (R-Ala.) told The Post. “It was just an exchange between two grown men.”

Sessions added: “I think it was definitely unity-building and everyone I’ve talked to said it was a very productive and good meeting.”

Even Sasse downplayed it.

“Senator Sasse went to today’s meeting ready to listen. Senator Sasse introduced himself to Mr. Trump and the two had a gracious exchange,” said a Sasse spokesman.

“Mr. Sasse continues to believe that our country is in a bad place and, with these two candidates, this election remains a Dumpster fire. Nothing has changed.”

But Trump did manage to land former rival Ted Cruz as a convention speaker.

Trump’s meeting with House Republicans was more subdued, earning him mostly accolades.

“It was a great meeting that will help Mr. Trump unify support behind him for his run,” said Long Island Rep. Lee Zeldin.