WASHINGTON—Rep. Matt Gaetz (R-Fla.) said Monday he will not leave Congress and denied that he “slept with” an underage girl, suggesting that accusations against him stem from political foes angry that he “loathes the swamp.” The Florida Republican, starting his fifth year in Congress, has been battling to preserve his political career since reports last week that he is under federal investigation for possible sex crimes. In a column in the Washington Examiner, a conservative news outlet, Gaetz predicted that “some of my feckless colleagues in Congress” will call for him to step down. “No, I am absolutely not resigning,” he wrote Monday. A high-profile ally of former President Donald Trump, Gaetz has so far received almost no public support from his congressional Republican colleagues and none from the former president. Gaetz, 38, is under scrutiny over whether he violated federal sex trafficking laws, including if he had sex with …