"We'll be opposing Roy Moore vigorously," McConnell told reporters after returning to the Capitol after a White House briefing on Iran.

McConnell's comments come after he warned, during an interview with the Associated Press shortly before Moore announced his bid, that he would "oppose him in every way."

Moore announced on Thursday afternoon that he would launch a bid for Alabama's Senate seat after previously winning the party's nomination in 2017, only to lose to now-Sen. Doug Jones (D-Ala.) in a contentious special election.

Moore lost the race after facing multiple accusations of pursuing sexual and romantic relationships with teenage girls decades ago when he was in his 30s; he has denied any wrongdoing.

National outside groups allied with McConnell had previously indicated they were leaving the door open to working to make sure Moore doesn't win the party's nomination for a second time. McConnell's comments mark the clearest indication that national Republicans intend to try to stop any potential momentum for Moore.

Jack Pandol, a spokesman for the McConnell-aligned Senate Leadership Fund, said Moore winning the GOP nomination "would be gift wrapping" the Senate seat for Democrats.

"I don't think it would help the president. I don't think it would help anybody running. He would … take a lot of oxygen out of there," Shelby said.

Asked if he thought GOP groups should actively campaign against Moore, he added: "I think a lot of them will."

Moore ran against the national GOP establishment during his 2017 Senate bid, and argued on Thursday that they will try to "smear" him.

“Why is there such a fear, such an anger to somebody running?” Moore said. “The mere mention of my name causes people to get up and arms in D.C.”

Rebecca Kheel contributed.

Updated at 6:04 p.m.