In recent conversations with aides and allies, President Trump often interjects with a question about his vice president: Does JD Vance have what it takes to go all the way?

He usually answers his own question: He’s not so sure.

It is not that Mr. Trump is abandoning Mr. Vance. He involves him in major decisions, has given him high-profile opportunities to position himself for 2028 and trusts the 41-year-old vice president to wage partisan warfare on his behalf. In a cabinet meeting this week, Mr. Trump compared Mr. Vance to Eliot Ness, the mob-busting federal agent, for working to ferret out fraud in mostly Democratic controlled states.

Mr. Trump has long conducted running focus groups on his closest aides, and appears to enjoy needling them and keeping them off balance as a way of asserting his dominance. Several people in the president’s inner circle have been subject to his quasi-public questioning of their performance and their future.

But when it comes to Mr. Vance, the stakes are higher. As the default front-runner for the Republican nomination and would-be inheritor of the president’s political movement, Mr. Vance’s fortunes ride to a substantial degree on the enthusiasm of the support he gets from Mr. Trump. And Mr. Trump’s regular polling of people on whether they prefer Mr. Vance or Secretary of State Marco Rubio has become one of the most closely watched early indicators of how power in the Republican Party might pass to the next generation.

When he conducts those polls in private, Mr. Trump often compares Mr. Vance’s performance to his own achievements. He has told several allies that Mr. Vance has never won a tough race without his help. (Mr. Trump’s endorsement got Mr. Vance over the finish line in a tight race for an Ohio Senate seat.) He has brought up the number of vacations Mr. Vance has taken as vice president. (Mr. Trump does not generally take them.)

He has repeatedly mentioned the vice president’s initial opposition to starting a war with Iran and has done so in front of Mr. Vance. (“I’m more of a peace person than you are — but I had to do it,” he has said to him.) The president has also questioned his decision to send a Vance-led delegation to a negotiation session in Pakistan that failed to end the war.