Aside from the Iowa caucus on Jan. 3, another date looms that might offer a measure of each presidential candidate's potential strength, and that date is Dec. 31, the deadline for filing fundraising reports for the final quarter of 2011.

A closely watched match-up will be the totals for Mitt Romney and President Obama. Romney reportedly hauled in $20 million in the past three months, the best fundraising sweep he  had all year. Obama's totals are not known, although he raised more than $40 million in both previous quarters and could do so again.

But those numbers won't tell the whole story about the amount of money spent to influence the outcome, campaign finance experts said, because of the rise of the super PACs that buy negative TV advertisements and aren't bound by the same rules regarding donation limits that candidates must follow.

The eventual GOP presidential nominee stands to benefit from that system. Super PACs like Karl Rove's Crossroads GPS  can buy negative ads attacking  Obama while the candidate  "stays positive," said Anthony Corrado, a Colby College professor of government who tracks the flow of money in politics.

"The rise of the super PAC has put less pressure on the Republican candidate to raise money in large parts," Corrado said. "While they're always interested in raising money, the kind of demands that used to be the expectation of needing to raise $50 million or $100 million by the time you get to year-end haven't been so intense."

That also means that the seven Republican presidential candidates can slog through several primary battles and not have to worry as much about missing out on money they could scoop up in a general election.

Romney, who is viewed  by many political insiders as the inevitable Republican nominee, has raised more money than his rivals vying for the party's nomination. Slater Bayliss, a Romney fundraiser in Florida who had supported Tim Pawlenty, said wealthy donors at this stage in the race are fueled by both an anti-Obama sentiment and a desire to nominate the most electable candidate.

Romney got a big boost last week when George H.W. Bush told the Houston Chronicle that the former Massachusetts governor is "the best choice for us." Bayliss, a former aide to Jeb Bush, said that Republicans who were still hoping that Bush would jump into the race saw that as the final sign that the former Florida governor was staying out of it, and they wrote their checks for Romney.

Bayliss described a "good migration of people" and said he anticipated that the elder Bush's semi-endorsement "frees up a million dollars off the sidelines."

"It's so much about momentum," Bayliss said. "These people, they talk a lot. If a couple of them are inspired, it has a cascading effect."

Jim Nicholson, a former Republican National Committee chairman who is working on Romney's team, said the campaign's tempo had kicked up recently as more donations pour in.

"I think that they've run a very smart campaign, and I think the momentum is now growing," said Nicholson, who was the secretary of veterans affairs under George W. Bush.

Republicans said that their candidate will be the underdog when it comes to fundraising against an incumbent president with a massive money machine that's already been tested in a presidential election. In addition to the $47 million that Obama raised for himself between April and June, and the $42 million he raised between July and September, he's also grabbed $65 million for the  Democratic National Committee in those six months.

Romney, meanwhile, raised $18.3 million between April and June, and $14 million between July and September. In those last three months, the GOP fundraising field was led by Rick Perry, who had just jumped into the race, with $17 million.

The Obama campaign has emphasized the importance of the Dec. 31 fundraising deadline to its supporters. A Tuesday night email from the Democratic Party signed by Vice President Joe  Biden said that fundraising totals for the final quarter of the year "is going to have a big effect on what we're able to do next year to re-elect this president." "This last week before the deadline will make a big difference," it said.

Romney supporters anticipate an outpouring of cash if he wins the Iowa caucus on Jan. 3. Even a second-place finish wouldn't be too harmful to his campaign. But if he places lower than that, donors might start to worry.

"If he ends up poorly in Iowa, then all of a sudden, that really puts a new focus on how important New Hampshire is," said Ray Washburne, a Romney backer in Dallas who was Pawlenty's chief fundraiser. "What are we pitching if he ends up in fifth place?"