Mr. Rubio, 44, may offer the field’s best case for “electability” because of his youth, charisma and Cuban heritage, and he is competing for many of the same supporters as Mr. Bush, including in their home state of Florida. A stronger Mr. Bush would have made it very difficult for Mr. Rubio to win the nomination, but Mr. Bush’s weakness has created an opening for Mr. Rubio that didn’t even seem to be there as recently as two or three months ago.

John Kasich, the governor of Ohio, also seemed unlikely to run until a few months ago. Like Mr. Rubio, and along with Chris Christie and Lindsey Graham, he could draw at least some supporters and voters from Mr. Bush. “I didn’t think I was going to be back up here again because, frankly, I thought Jeb was just going to suck all the air out of the room,” Mr. Kasich said two weeks ago in New Hampshire, “and it just hasn’t happened.”

Mr. Bush’s fund-raising, with a goal of $100 million for his super PAC, is impressive. But it’s hard to say just how impressive it really is in an era of unlimited contributions, when even a candidate like Ted Cruz can raise more than $30 million for his super PAC in a few weeks. And in recent days, there have been reports that Mr. Bush might fall well short of early fund-raising expectations — a further sign of weakness.

Even if his fund-raising is as strong as he hoped, there is no reason to expect it will be decisive: Mr. Romney barely won pivotal states like Ohio and Michigan in the 2012 primaries, despite an overwhelming financial and organizational edge over a candidate, Rick Santorum, who was not nearly as strong as Mr. Bush’s competition is today.

Perhaps most important, it’s surprisingly hard to find prominent elites who support Mr. Bush — aside from a spurt of donors and high-profile aides who joined his team a few months ago and late last week. When my colleague Peter Baker recently surveyed 120 people who worked for President George W. Bush, only about 25 responded to say they supported Jeb Bush. You won’t find many Republicans and conservatives praising him on television or in print. So far, Mr. Bush has received no formal endorsements from outside Florida, and nothing is a better indicator of primary strength than endorsements.

Mr. Bush, of course, might eventually generate a surge of endorsements. Fund-raising strength usually correlates with endorsements, even if a handful of candidates with unusually close ties to Wall Street and strong fund-raising performances but poor results — like Phil Gramm, chairman of the Senate committee on banking, and the former New York mayor Rudy Giuliani — have soured analysts on this metric.