WASHINGTON — Donald Trump Jr. and Republican Sen. Jeff Flake are engaging in a war of words over Twitter.

The Arizona lawmaker, one of the most vocal GOP critics of President Donald Trump, tweeted Tuesday about his bill that would bar anyone convicted of domestic violence in a criminal or military court from purchasing a weapon.

The gunman who killed 26 people at a Texas church Sunday was able to buy weapons because the Air Force failed to submit his criminal history to the FBI as required by law.

Trump Jr. responded to Flake, “Incredibly proactive considering that law has been on the books since the mid 90s.”

Flake answered, “If being proactive means closing the #DomesticViolenceLoophole exploited by the #SutherlandSprings Texas shooter, you’re right.”

The president’s eldest son then tweeted: “Wrong. Govt employees, like you, failed to do their job. He did a year for abuse & fracturing his kids skull. Should not have had a gun!”

Flake announced last month that he would not seek another term, delivering a forceful speech on the Senate floor in which he denounced the GOP president.

“We were not made great as a country by indulging in or even exalting our worst impulses, turning against ourselves, glorifying in the things that divide us and calling fake things true and true things fake,” he said.

Writing a bill w/ @MartinHeinrich to prevent anyone convicted of domestic violence – be it in criminal or military court – from buying a gun — Jeff Flake (@JeffFlake) November 7, 2017

Incredibly proactive considering that law has been on the books since the mid 90s. https://t.co/li2bSEW0LE — Donald Trump Jr. (@DonaldJTrumpJr) November 7, 2017

If being proactive means closing the #DomesticViolenceLoophole exploited by the #SutherlandSprings Texas shooter, you're right. https://t.co/8S89dgaTuK — Jeff Flake (@JeffFlake) November 7, 2017

Wrong. Govt employees, like you, failed to do their job. He did a year for abuse & fracturing his kids skull. Should not have had a gun! https://t.co/olpA7Sgx0K — Donald Trump Jr. (@DonaldJTrumpJr) November 7, 2017

Flake and Tennessee Republican Sen. Bob Corker have decided against running for re-election, which has given them more leeway to criticize the president.