Speaker Mike Johnson’s success at squeezing a Republican budget resolution through the House this week, after his brush with defeat just minutes beforehand, underscored his shaky hold on his unruly majority — and how heavily reliant he is on President Trump to keep his members in line.

He had no Plan B if the budget plan were to fail, he told reporters on Tuesday night as he entered the chamber for a vote that he had a very solid chance of losing. And in the end, Plan A relied on a big handhold from the president, who made calls that ultimately flipped the few Republicans who were standing in the way of advancing his domestic agenda through what he has called “one big beautiful bill.”

The president was “a big help,” Mr. Johnson said after the vote. That may have been an understatement. The moment reflected how Mr. Johnson, who has already made himself subservient to Mr. Trump, has become dependent on the president to manage the punishing math of his tiny majority and the erratic personalities and uncompromising viewpoints of his rank and file.

It was Mr. Trump who talked to Representative Tim Burchett, the Tennessee Republican who was one of the final members threatening to vote against the bill, for 15 minutes before he finally voted “yes.” Another of the last holdouts, Representative Victoria Spartz, Republican of Indiana, was spotted by her colleagues crying as she spoke on the phone with Mr. Trump in the cloakroom just off the House floor.

“He’s on board to get some great things done on health care,” she told reporters later. “I trust his word.”

The vote to adopt the budget on Tuesday night followed a similar pattern as the one that elected Mr. Johnson speaker in January. He had been on the verge of defeat on the first ballot, but was pulled over the finish line after Mr. Trump interrupted his golf game to lobby holdouts by phone.