Their reservations have been reinforced by McCain and his advisors, who have counseled fundraisers to keep their powder dry until the field develops further, according to people familiar with the conversations.
Brooke Buchanan, a spokeswoman for McCain, said the senator had not explicitly advised people whether to get involved in the race, but had told friends who have asked that they should carefully review the candidates before signing on with anyone.
For now, some of the party's most prolific fundraisers -- including Michigan real estate developer Michael Kojaian, Virginia home builder Dwight Schar and Howard Leach, former ambassador to France -- all remain free agents.
Leach said he expected many donors to stay on the sidelines until a frontrunner emerged.
"Once there is identification of a clear candidate, there'll be tremendous energy," he said. "It'll move very fast and very effectively."
Tom Hamburger in the Washington bureau contributed to this report.