As the F.B.I. has added payback to its portfolio, Republican lawmakers like Senator Charles E. Grassley have emerged as a clearinghouse for leaks and whistle-blowers.
The president urged Representative Julia Letlow of Louisiana to run against Mr. Cassidy, in a move that is likely to further complicate his relationship with Senate Republicans.
Republicans have cheered the raid that ousted Nicolás Maduro but toiled to square their longstanding views with President Trump's unclear and rapidly evolving strategy.
