WASHINGTON, May 16 (Reuters) - U.S. President Donald Trump threatened a primary challenge on Saturday for hardline conservative Representative Lauren Boebert, until now a staunch Trump ally, after she campaigned for maverick Representative Thomas Massie in his Kentucky district. "Boebert is campaigning for the Worst 'Republican' Congressman in the History of our Country, Thomas Massie, of the Great Commonwealth of Kentucky, and anybody who can be that dumb deserves a good Primary fight!" Trump wrote on Truth Social. Boebert, a Colorado congresswoman, responded with a post on X: "Yes, I saw the President’s post. No, I’m not mad or offended....