President Trump was angry.
“He was mad as a murder hornet,” Senator John Kennedy, Republican of Louisiana, said of the president’s mood during a closed-door lunch on Wednesday with G.O.P. senators.
According to people who attended, Mr. Trump arrived on Capitol Hill with a long list of grievances against members of his own party.
He lamented Republicans’ failure to push through a bill to impose new restrictions on voting that he has called key to the G.O.P.’s chances of winning the midterm elections, vented about years-old legal disputes, complained about the G.O.P.’s refusal to blow up the Senate filibuster and pressed the senators on stalled judicial nominations.
But what appeared to enrage him the most were the four Republicans who had crossed party lines a day earlier to back a war powers resolution directing him to halt military operations against Iran or seek congressional authorization to continue. Their support allowed the measure to be adopted, delivering a rare bipartisan rebuke of his handling of the war.
“Why would anybody vote for the War Powers Act?” Mr. Trump demanded to know.
For a few moments, senators sat silently, taken aback by the president’s gruff question. Then Senator Bill Cassidy, the Louisiana Republican who lost his primary last month to a Trump-backed opponent, spoke up with a spirited critique of the president’s handling of the war and his lack of consultation with Congress about it.
“I stood and said, ‘You have not told the American people what’s going on,’” Mr. Cassidy recounted afterward. “It was supposed to last four weeks; it’s lasted four months. Our original objectives have not been achieved. And I want to know what’s going on.”
Mr. Cassidy, one of the four Republicans who voted for the war powers resolution, said he grew increasingly frustrated during the exchange as he noted that senators had yet to receive a comprehensive briefing on the conflict, which Mr. Trump has carried out without congressional authorization.
The president did not appreciate being questioned, and the exchange quickly became heated, according to Mr. Cassidy and other attendees, devolving into a shouting match.
“You lost the election,” Mr. Trump shouted at Mr. Cassidy, whom he called a “loser.” The two traded testy remarks.
“If someone tries to bully me,” Mr. Cassidy said afterward, “I ain’t gonna put up with that.”
“I make no apologies for standing up to the president,” he added. “I’m sticking up for the American people, even if I’m speaking to the president.”
Mr. Trump’s frustration reflected more than a single vote. There had been weeks of tension between him and Senate Republicans on matters of politics and policy. And just before the meeting on Wednesday, the president had abruptly scrapped a signing ceremony for a major bipartisan housing bill, scuttling what G.O.P. leaders had hoped would be a triumphant moment to highlight their efforts to address high costs ahead of the midterm elections.
And while party leaders remain firmly aligned with Mr. Trump on most issues, the war in Iran has exposed fissures and prompted some G.O.P. senators, who initially deferred to the president almost unquestioningly, to begin seeking more of a role for Congress.
During the lunch, Mr. Trump said he was particularly angry with Senator Dave McCormick, Republican of Pennsylvania, for missing Tuesday’s vote, given that the resolution was adopted by a narrow margin. (Mr. McCormick was traveling with the president at the time, and his presence to vote “no” would not have been enough to defeat the measure.)
Mr. Trump also lashed out at Senator Lisa Murkowski, Republican of Alaska, who was part of the bloc of Republicans that supported the resolution, calling her a “horrible person,” according to senators in attendance.
Mr. Kennedy said the president’s anger was understandable, given the timing of the bill’s passage.
“He was very upset about the war powers vote,” Mr. Kennedy said. “He was in the middle of pretty heavy-duty negotiations and he had to stop and explain to the Iranian negotiators why it was a meaningless vote. And that clearly upset him. And I don’t blame him, frankly.”
“The president is very, very candid,” Mr. Kennedy added. “He told us exactly how he felt, and I appreciate it.”
“He’s right to be mad, in my opinion,” Mr. Kennedy said.
Emerging from the meeting, Mr. Trump put a cheerful face on the bitter gathering, calling it “a really great meeting.”
“We’re very proud of the party,” he said. “We like our leader. We like everybody, really, in the room.
“I don’t like a few people,” Mr. Trump added, “but that’s OK.”
