Some of the bundlers and donors said they had held back, in part, because the field was the strongest they had seen in years, with several viable contenders representing the party’s different generational and ideological segments.

Unlike in 2012, when Mr. Romney dominated fund-raising even as he fought off a series of insurgencies by more populist candidates, the affections of many donors in 2016 are divided among three or four candidates. Others are quietly weighing the impact of Donald J. Trump, who has jumped to the lead in some national polls despite raising almost no money from the party’s establishment.

“I haven’t committed to anyone at this point, and I’m not on the verge of committing to anyone,” said Paul E. Singer, a hedge fund manager who is among the most sought-after Republican bundlers in the country, at an investment conference last week. “I think there are a number of candidates that are smart, solid, good potential leaders, leaders and potential leaders.”

The slow recruitment of major donors and bundlers is also a function, several donors and Republican leaders said, of the candidates’ early emphasis on raising money for “super PACs,” which tend to be funded by a much smaller pool of extremely wealthy donors.