But Mr. Romney, who strongly considered a 2016 presidential campaign of his own before bowing out as Mr. Bush made a show of fund-raising force, is not ready to make an endorsement, according to three people close to him,

His advisers are also said to be split as to whether he should do so. But Mr. Romney has been incensed by the campaign that Mr. Trump has run, according to those close to him, though he has only occasionally spoken out publicly, as he did after Mr. Trump praised President Vladimir V. Putin of Russia. “Important distinction: thug Putin kills journalists and opponents,” Mr. Romney wrote on Twitter. “Our presidents kill terrorists and enemy combatants.”

Mr. Bush’s backing will most likely be tougher to secure. He and Mr. Rubio clashed heatedly over the last few weeks of the campaign, and Mr. Bush was plainly wounded by the experience of being upstaged by a candidate who was once his protégé.

In his remarks on Saturday, Mr. Rubio praised Mr. Bush as a visionary governor and a good man. But it may take a more concerted effort at fence-mending to persuade Mr. Bush to join forces with Mr. Rubio.

Another obstacle for Mr. Rubio is the presence of Gov. John Kasich of Ohio. The two are fighting for some of the same voters, and Mr. Rubio’s campaign openly pushed for Mr. Kasich to leave the race in a memo sent to reporters on Sunday.

“John Kasich has no path to the nomination,” Mr. Rubio’s advisers wrote, asserting that there was no way for Mr. Kasich to accrue the needed delegates.