President Trump's meeting with Republican senators on Wednesday turned testy, and at one point, he sternly told Louisiana Sen. Bill Cassidy to sit down, after questions arose about the War Powers Resolution, according to sources directly familiar with the meeting.

Cassidy had been trying to respond to the president, telling the president he should sit down — but Cassidy later relented, saying he would be seated, but not at Mr. Trump's behest. 

"He raised his voice. I lost my temper. That's not appropriate. It's the Irish in me, but I again matched his tone and his volume," Cassidy told reporters after the meeting.

Earlier this year, Mr. Trump supported Rep. Julia Letlow in the Louisiana Republican Senate primary. Cassidy did not advance to the runoff, likely ending his political career, a point Mr. Trump highlighted in his remarks.

"I make no apologies for standing up to the president," Cassidy said. "I am sticking up for the American people, even if I'm speaking to the president."

The more-than-hourlong meeting with Mr. Trump focused mostly on the Iran war and the War Powers Resolution. On Tuesday, the Senate approved a Democrat-led resolution to keep the president from ordering further military action in Iran. Four Republicans voted in favor of the concurrent resolution, which is symbolic and does not carry the force of law.

A source directly familiar with the meeting told CBS News Mr. Trump expressed his discontent with Republicans, including Cassidy, who had worked with the Democrats on the resolution. 

The president also shared his disdain for Alaska Sen. Lisa Murkowski. "It was very awkward when she walked in right after he said that," the source said.

Murkowski arrived at the meeting late, telling reporters that she had a previously scheduled event. 

Leaving the lunch, GOP senators described a "lively" and "spirited" meeting. Sen. Tommy Tuberville of Alabama told reporters the tone was "half-time talk." Sen. Kevin Cramer of North Dakota said the president talked about being undermined by the Republicans who supported the Iran resolution and "expressed some disappointment."

"He wasn't as upset as I probably would have been about it," Cramer said. 

"I think we had a really great meeting. We're very proud of the party. We like [Majority Leader John Thune]," Mr. Trump said. "We like everybody really in the room. I don't like a few people, but that's okay. I think you know who they are."

Shortly before arriving on Capitol Hill, the president abruptly canceled plans to sign a bipartisan housing bill. Mr. Trump wrote on social media that he would only sign it once Congress passed the SAVE America Act, a pet project of the president which, among other provisions, would require voters to show proof of citizenship when they register. Senate GOP leaders have repeatedly said the bill does not have enough support in the upper chamber to pass. 

The president told senators that Republicans must win in November, and need Republican voters to show up. Without passing the Save America Act, the president said, they won't show up. 

The president suggested eliminating the filibuster in order to pass it, which would lower the threshold for passage from 60 votes to a simple majority. Republican Senate leadership has so far resisted Mr. Trump's entreaties to do away with the filibuster, and doing so would not guarantee its passage, since a handful of Senate Republicans in the slim GOP majority remain opposed. 

A source familiar with the meeting said Mr. Trump's parting message as he left was that the SAVE America Act has to get done.

"The president closed by preaching unity," said Texas Sen. John Cornyn, who also lost his Republican primary to a Trump-backed candidate. "But he spent the prior hour talking about things which were not exactly unifying."