That has become Mr. Massie’s battle cry these days as the only Republican in the country centering his re-election bid unapologetically on his willingness to buck his own party, even if it means defying President Trump. He has been singled out by Mr. Trump as “disloyal” and “a complete and total disaster” after leading the charge within the G.O.P. to release the Epstein files. A thorn in the president’s side on everything from federal spending to the war in Iran, Mr. Massie is now embroiled in the toughest fight of his political career, a contest set to be the most expensive congressional primary in the nation.