As a congressman who led the first impeachment of President Trump, Mr. Schiff relished his role in the resistance. Now a senator, he must protect his state’s interests at a perilous time.

The Bay Area politician, known for her antiwar position and breaking barriers in Congress, just left the House after more than two decades.

The president hasn’t yet decided what he will do, he said, acknowledging that the makeup of Trump’s new administration could influence his decision.