Mr. Rubio’s allies, who have been pushing for Mr. Romney’s endorsement, believe that his eventual backing is all but certain. Some thought the endorsement was imminent on Saturday.

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.