The president moved quickly to cancel the CBP One app, which allowed migrants to schedule appointments to gain entry into the United States, turning away potentially tens of thousands of migrants.

Elected officials across the party are engaging in a balancing act, signaling they have heard voters’ demands for change while grappling with when to oppose Donald Trump.

The White House scrambled to get a message to President Vladimir V. Putin of Russia last year after U.S. intelligence agencies said a Russian military unit was preparing to send explosive packages on cargo planes.