His economic expertise and experience as governor could balance John McCain's strength on foreign policy and long tenure in the U.S. Senate. He is a decade younger than the 71-year-old senator and looks younger still.

But Mitt Romney's most valuable role as McCain's running mate could be as rainmaker.

Romney was not only the wealthiest candidate in the 2008 Republican primary field; he also outraised every other contender while he was in the race. And in a year when Republicans are confronting an unprecedented Democratic money machine, Romney's fund-raising prowess could make him especially attractive.

Earlier this month, Romney urged his finance team to contribute to McCain; many of his top donors attended a fund-raiser with President Bush at Romney's vacation home in Deer Valley, Utah on Wednesday night.

In interviews, major Romney donors and fund-raisers said the financial spigots would open much wider if Romney were actually on the ticket.

"There is no doubt in my mind that if Senator McCain selected Governor Romney, there would be a tremendously positive response from those that may not yet be engaged on behalf of Senator McCain," said Thomas R. Tellefsen, a California investor and former national finance chairman for Romney's presidential campaign.

Tellefsen said some Romney donors had not enlisted to help McCain yet because they were new to politics and had a personal connection to Romney. Other supporters said some donors may not be as enthusiastic about McCain as they are about Romney, but a joint ticket might mobilize them.

"You can raise a lot more money if you're passionate about what you're doing," said Ted Welch, a Republican fund-raiser from Tennessee who was a national finance co-chairman for Romney, and who said he raised $155,000 for McCain in the last week.

Some Romney supporters were reluctant to speak publicly about the financial advantage Romney could bring to the GOP ticket, fearing that their comments would be misinterpreted as pressuring McCain to pick him.

"The fact remains that Romney supporters are demonstratively better givers and more aggressive givers" than supporters of other primary candidates this year, one Romney donor said. "But nobody's going to come out and say anything to make it look like they're putting Senator McCain into a corner. .. John McCain is a fiercely independent individual who's going to pick what his gut tells him to pick.".

Other supporters rejected the notion that Romney's wealth or fund-raising ability should be a factor.