While Mr. Bush had $7.6 million in the bank and the “super PAC” supporting him had an additional $58.6 million as of Jan. 1, Mr. Kasich had just $2.5 million and Mr. Christie a paltry $1.1 million. And neither Mr. Christie nor Mr. Kasich, both of whom have spent the vast majority of their time in New Hampshire, has much in the way of organization in South Carolina.

Mr. Bush, by contrast, has an active network of supporters there and has tapped into Mr. Graham’s organization. And perhaps most important, the super PAC supporting Mr. Bush has shown a willingness, to put it mildly, to spend millions of dollars against his rivals, particularly Mr. Rubio, whom it has spent more than $20 million attacking.

The prospect of the super PAC maintaining its assault on Mr. Rubio after the New Hampshire primary leaves many Republicans uneasy about Mr. Bush’s intentions. And Mr. Rubio’s supporters, cognizant of these concerns, began moving on Tuesday to stoke these fears.

“If Jeb doesn’t do well there, and he’s still got millions and millions to spend, does he stay around?” mused Senator Cory Gardner, Republican of Colorado, a Rubio backer. “Who knows.”

But, Mr. Gardner noted, “more money has been spent attacking Republicans by fellow Republicans in this election than any cycle in the history of the party.”

It is not just supporters of Mr. Rubio who are dismayed. Some of Mr. Bush’s major donors, including those with long ties to the family, are also growing impatient, hoping he will not go past New Hampshire absent a strong finish here.