She warned her colleagues that infighting among Republicans could give Democrats a good chance of capturing her seat in 2014, when she faces reelection, according to The New York Times, which reported on the private argument.
Sen. Rob Portman (R-Ohio) stood up at the meeting to express frustration that Collins had to use money he donated her to fend of attacks from the National Association of Gun Rights.
Paul, feeling attacked, stormed out of the meeting, according to The Times.
The senator on Wednesday said he does not plan to support primary challenges against any of his Republican Senate colleagues, although he might lend his support to primary challengers running for open seats.
“At this time I don’t have any plans to oppose any incumbent Republican,” Paul said.
The National Association for Gun Rights has targeted other Republican senators in the campaign to oppose expanded background checks, including Sen. Johnny Isakson (Ga.) and Chuck Grassley (Iowa).