Donald Trump’s attempts to win over skeptical Congressional Republicans grew tense Thursday when Arizona Sen. Jeff Flake confronted the presumptive GOP presidential nominee over his incendiary comments about Sen. John McCain, according to a source familiar with the closed-door meeting.

Flake stood up for his fellow Arizona lawmaker by introducing himself to Trump as “the other senator from Arizona, the one who didn't get captured,” referring to Trump’s criticism of McCain's time as a prisoner of war during the Vietnam War.

"Listen, I'm not part of this never Trump movement," Flake said, according to the source, "But I'm in a very uncomfortable position where I can't support you yet."

The interaction, first reported by the Washington Post, follows a number of highly critical statements Flake has made about Trump’s controversial comments, specifically on Muslims and immigration. The Arizona senator emerged from the meeting saying he can't support Trump "given the things he said."

Trump responded by threatening to “hit hard” at Flake, according to the source. Trump also told Flake he would not win re-election, to which Flake told him he was not up for re-election this year.

More than 40 senators attended the meeting which was aimed at building GOP unity heading into the convention later this month. But other awkward moments included senator and Never Trump leader Ben Sasse's appearance at the meeting. Trump seemed surprised to see Sasse at the exchange, according to a source familiar with the meeting.

"Ben has hit me hard, he's been tough on me," Trump said, accoring to sources. But Trump added, "I like him and I'm glad he's here."

Let our news meet your inbox. The news and stories that matters, delivered weekday mornings. This site is protected by recaptcha

Trump was generally well received by House Republicans after meeting with them earlier in the day. House Speaker Paul Ryan said Trump “did a great job engaging with our members and I think our members appreciated it.”

“Today’s meeting was positive and productive and these characterizations, attributed to unnamed sources, are wholly inaccurate," Trump campaign manager Paul Manafort said in a statement. "The conversation was very positive and substantive. The Members were in total agreement with Mr. Trump of the need to unite the party."

Trump’s visit to Capitol Hill was meant to serve as chance for the party’s nominee to directly appeal to Republican lawmakers still skittish about his candidacy. Members who attended Trump’s meeting with the House Republican Conference described the presumptive Republican presidential nominee as measured and said he attempted to explain some of the controversies that have surrounded his campaign, like his positive comments about former Iraqi dictator Saddam Hussein’s ability to kill terrorists.

“We get taken out of context all the time, and I think his point was to put it into context so that people understood the context on which he was speaking about getting tough on terror,” Ryan said after during his weekly press conference following the meeting.

Related: Trump Can't Stay on Message, and It's a Problem for the GOP

House Republicans who spoke to the media following the meeting had largely positive reviews and were optimistic that Trump can unify the party.

“What you saw today was Donald trump, the next president of the United States, supercharge the Republican conference one standing ovation after another,” said New York Rep. Chris Collins, Trump’s first Congressional endorsement.

“What I felt in the room up there is a great sense of unity, us coming together. It appears to me we’re on the same page, that members of the Republican conference and Donald Trump…We’re all moving in the same direction,” said Rep. Bill Shuster, who also has endorsed Trump.

Trump supporters emerged from the meeting largely shying away from talking about Trump’s tweet featuring a six-pointed star that was criticized for being anti-Semitic, which he defended during a rally Wednesday night.

Ryan, who previously said there is “no place in a presidential campaign” for those types of social media posts, brushed aside a question about it during his news conference.

“You think I’m going to comment on every tweet?” he asked.

“I didn’t even see the thing when I commented on it, so I’m just not going to get into that tweet talk,” he added.

A source in the room told NBC News that Nevada Rep. Crescent Hardy asked Trump the first question, wanting to know how the real estate mogul would appeal to Hispanic voters. Trump told members, like he has said on the campaign trail, that Hispanic voters support him because of his plans to grow jobs in the U.S.

And even members who have been critical of Trump, like New York Rep. Peter King, had positive reviews of the meeting.

“I would honestly say there was not one negative moment, there was no awkward moment, it was all positive, it was very united,” King told reporters. “And we'll see where it goes. But today, if today is the way it's gonna be, it's gonna be an extremely effective campaign.”

Trump also met one-on-one with former primary rival Ted Cruz while in Washington. A statement released by Cruz after the meeting said the Texas senator agreed to speak at the GOP convention but "there was no discussion of any endorsement."