Washington — GOP Sen. Bill Cassidy of Louisiana on Wednesday dropped his support for a Democrat-led effort to restrict President Trump's war powers in Iran after receiving a briefing at the White House — a briefing he said he secured with a note slipped to the president's envoy to the Middle East following a heated exchange with Mr. Trump behind closed doors.  

"In one sense, I actually accomplished the mission of what I needed to do," Cassidy said in an interview Thursday for "Face the Nation with Margaret Brennan." 

Cassidy and the president sparred at a testy Senate Republican lunch meeting on Wednesday, where Mr. Trump expressed his discontent with four GOP senators, including Cassidy, who had supported a resolution aimed at reining in the president in Iran a day prior. The Louisiana Republican described losing his temper at the meeting, after the president raised his voice.  

In an interview with Margaret Brennan that will air on Sunday, Cassidy said that after his exchange with the president, he passed a note to Middle East envoy Steve Witkoff, saying he would consider changing his vote.

"Steve, I would consider changing my vote, but I've been voting yes because I've not been briefed," Cassidy recalled writing to Witkoff. "He said, 'Call me back in the hour and let's have a briefing.' We had it last night."

After the briefing, Cassidy opposed the latest attempt by Democrats to advance a war powers resolution Wednesday night in a key vote that helped Republicans to defeat the effort. 

Cassidy, who lost his Senate primary to a Trump-backed challenger in May, outlined that his rationale for supporting the previous war powers efforts had been "because we were not being briefed."

"We could be the Senate, the Congress, or the United States, and I felt important that we be briefed," he said. "I agreed with the president's original goals, those were not being achieved by my perception, and so before I could say, 'okay, everything's hunky dory,' I said I need to be briefed."

When asked what he heard at the briefing that changed his mind, the Louisiana Republican said if the original objectives were to degrade Iran's nuclear capability, its ballistic missiles and conventional warfare capabilities, it appeared to him that the objectives "can be reached." 

"Now we have to trust but verify, but as they laid it out, they have a plausible plan by which to achieve those, and that's what I was interested in," Cassidy said. 

Cassidy, a medical doctor, said he's always "going to try and get as much information as possible to come to the truth of someone's diagnosis and the truth of how to treat that problem."

"You deny me that information, and I'm going to be frustrated, because my job is to serve with the information I have before me," he said. "I take that same ethic to public service."

Cassidy said "If you're not telling me answers, I'm going to push for those answers." 

"So, when the president was berating the four people that voted for the War Powers Act, frankly, I'm not there to be berated," he said. "And the president wasn't invited to dish out verbal abuse."

During the meeting with the president, Cassidy said he raised his hand and asked whether the president was asking a rhetorical question on why the small group of Republicans had supported the Iran measure. 

"He goes, 'I'm really interested.' I stood up and I said, 'this is why,' and I listed those objectives that I did not see being achieved, and how the kind of endpoint of the war kept stretching out longer and longer," Cassidy said. "He began to speak over me. Unfortunately, I raised my volume to match his."

Cassidy said "I shouldn't have lost my temper, nor should he, but you know, my wife will tell you every now and then, my Irish temper gets the best of me."

"But point being, I needed to know. I need to know to serve my people and my state and my country," he said. "As it turns out, I got a briefing afterwards."