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 military court from purchasing a weapon.

The gunman who killed 26 people at a Texas church Sunday was able to purchase 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 tweeted prior to a Capitol Hill news conference with Sen. Martin Heinrich, D-N.M., in which the two lawmakers announced the introduction of a bill that would require the military to identify cases of domestic violence offenders and submit the names to the database of prohibited gun buyers.

Flake explained that the military has no defined charge under the Uniform Code of Military Justice for domestic violence. Consequently, cases fitting those circumstances went unreported in what he described as an "egregious oversight."

The federal Gun Control Act of 1968 prohibits firearm sales to someone who has been convicted of a domestic violence offense. However, since 2007, the military has submitted only one domestic violence offender to the database, the lawmakers said.

"Unless we are to assume there had been only one case of domestic violence in the military over that period of time, then we have a problem, we have a loophole here that needs to be closed," Flake said.

He said the proposed legislation would force the military to filter out from other assault cases those convictions that would be considered a domestic violence crime in state court. The military would then be required to report the name of the offender to the Department of Justice.

Prospects for the legislation were unclear as the Republican-led Congress has been reluctant to vote on any bill tightening gun controls.

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.

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