Kinzinger, a member of the Jan. 6th House Committee, posted a letter he says was sent to his home, and here it is ... "You are one stupid c***. That pimp you married not only broke his oath, he sold his soul. Yours, and Christian's too! Adam's activities have not only hurt this country, but countless patriotic and God-fearing families. Therefore, although it might take time, he will be executed."

Then the writer goes on to attack Adam's wife and son ... "But don't worry! You and Christian will be joining Adam in hell too! We find it blasphemous that you name the son of the devil Christian!"

Kinzinger, who is not running for re-election, appeared on ABC's "This Week" Sunday and said he believed Trump's involvement in the insurrection rose to a level of criminality.

In posting the letter, Kinzinger blamed the tone of the GOP for people like this letter-writer going nuts ... "The Darkness is spreading courtesy of cowardly leaders fearful of truth. Is the what you want @GOP? Pastors?"

BTW ... along the same lines, a GOP candidate named Eric Greitens, who's running for the U.S. Senate in Missouri, appears in a campaign ad with gun in hand, saying he's going after RINO's ... Republicans In Name Only.