Michael Avenatti and Donald Trump Jr. have been at each other's throats on Twitter since Monday.

While at a lunch with an NBC News reporter on Tuesday, Avenatti challenged Trump to a mixed-martial-arts charity fight.

Avenatti confirmed the offer, saying he wanted any money raised from such a fight to go toward rebuilding Puerto Rico and to a sexual-assault charity.

On Monday, Trump joked that Avenatti would be running for president in 2020 with the incarcerated politician Anthony Weiner; Avenatti replied that Trump would be "indicted" soon and pass Weiner in jail.

Michael Avenatti, the lawyer who has become a vocal critic of the Trump administration, has challenged Donald Trump Jr. to a three-round mixed-martial-arts charity fight, saying it "would be for two great causes."

The NBC News reporter Dylan Byers was with Avenatti at the Vanity Fair summit on Tuesday, when Avenatti proposed the matchup. Byers tweeted about the proposal, and Avenatti replied, confirming it.

"This would be for two great causes. I'm in," Avenatti tweeted.

In a follow-up tweet, Avenatti proposed that proceeds from such a fight would be "split 50% to rebuilding Puerto Rico and 50% to @ RAINN."

RAINN, the Rape, Abuse & Incest National Network, runs a 24-hour hotline for sexual-assault victims.

The tweet also said: "Just waiting on Bif to confirm. # Basta."

Donald Trump Jr. John Moore/Getty Images

Trump and Avenatti have shared barbs online this week, with Avenatti tweeting on Tuesday the president's eldest son would "be indicted" on federal charges very soon.

The day before, Trump mocked Avenatti on Twitter, saying: "Who else is excited about Avanetti/Weiner 2020?!? It's gonna be Carlos Dangerous."

Anthony Weiner, the former Democratic congressman from New York, is serving a 21-month prison sentence for sending obscene material to a minor, though CNN reported Wednesday that he was expected to get out three months early.

In response to Trump's tweet, Avenatti wrote: "If I were you, the last thing I would be doing is referencing other people getting out of federal prison. Because after you are indicted, you will likely be passing them on your way in. BTW, they don't have silver spoons or gold toilets in the joint. Buckle up Buttercup."

Avenatti was the attorney who submitted a lawsuit on behalf of the adult-film actress Stormy Daniels to repeal a 2016 nondisclosure agreement made with Donald Trump.