After President Trump ousted Mike Waltz, his national security adviser, on Thursday night, he settled on someone less hawkish on Russia and willing to remain in lock-step with his foreign policy approach to Iran, Gaza and China.

He didn’t have to look far.

By making Marco Rubio the top foreign policy adviser in the West Wing, in addition to his main day job as secretary of state, Mr. Trump turned to a one-time political rival who has spent the first three months of the administration as a loyal, globe-trotting foot soldier and a reliable echo of the president’s agenda.

Now Mr. Rubio will help run that agenda from inside both the White House and the State Department headquarters — even as the president’s longtime friend, Steve Witkoff, remains the chief negotiator, in charge of finding an end to the wars in Ukraine and Gaza and reaching a deal with Iran on its nuclear weapons program.

Leslie Vinjamuri, the director of the U.S. and the Americas Program at Chatham House, a London-based research institute, said Mr. Rubio is “willing to align and to follow with where Trump is. What we’re getting, throughout this administration, is: Loyalty comes first, loyalty to the man, loyalty to the mission.”

But by consolidating so much foreign policy power in one person, she added, Mr. Trump risks losing someone who might provide him with different policy perspectives or competing advice.

“You just reduce the number of potential points for somebody saying, ‘Actually, whoa. Look what just happened,’” she said. “‘Look at this piece of information that flies in the face of what we suspected.’”