Bill Cassidy

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.”

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