BRANCHBURG, N.J. — President Trump, back at the Bedminster, N.J., golf getaway where he decided to fire James B. Comey, took his harshest shot yet against his former F.B.I. director on Sunday morning, calling him “cowardly” and vowing to find out if he had leaked any more sensitive information.

“I believe the James Comey leaks will be far more prevalent than anyone ever thought possible,” Mr. Trump wrote in one of his patented early morning Twitter tirades — fulfilling the expectations of staff members who had feared he would lash out after spending a sunny weekend watching replays of Mr. Comey calling him a liar.