• Rex Tillerson is said to have learned of his dismissal by being shown President Trump’s tweet naming Mike Pompeo, the C.I.A. director, as U.S. secretary of state.

Mr. Pompeo and Gina Haspel, his designated successor at the helm of the C.I.A., are policy hawks. (Ms. Haspel had a leading role in the torture of detainees.) Their appointments await Senate confirmation.

Meanwhile, Mr. Trump ventured to California, the blue bastion of liberal resistance, with a combative message on immigration.

A by-election in Pennsylvania has become an unexpectedly narrow test of whether Democrats can make inroads with voters who supported Mr. Trump in 2016.