On the surface, Barack Obama enjoys a substantial political money advantage over John McCain in the White House contest.

Obama's decision to opt out of the presidential public financing system may have drawn howls from reform advocates and editorial boards, but it could free him to outspend McCain by 2-1 or more. McCain, having embraced public money for the general election, now faces a spending cap of just under $85 million. If Obama maintains the extraordinary fundraising momentum that's netted him close to $300 million so far, he'll have no trouble swamping McCain financially.

Obama, moreover, is backed by labor unions and other well-heeled liberal groups poised to spend record sums on the race. The Service Employees International Union and the AFL-CIO alone have announced plans to dole out upwards of $138 million on this election. GOP-friendly activist groups, by contrast, have yet to demonstrate that kind of heft.

Election after election, political scientists note, has demonstrated that money isn't everything.

But Obama's apparent money edge is offset by several drawbacks, both practical and political. McCain has already started raising money jointly with the Republican National Committee, which is dramatically outperforming its Democratic counterpart. The RNC's receipts through May 31 totaled more than $147 million, public records show, compared with just $77 million for the Democratic National Committee. Records also show the RNC with $53.5 million cash on hand, more than 10 times the DNC's $3.9 million.

McCain's own primary fundraising [PDF] enjoyed a much-needed boost in May, putting his $21.5 million almost on a par with the $22.7 million that Obama took in that month. And Democrats as a whole -- including liberal 527 groups -- solidly outspent Republicans in 2004, only to face defeat on Election Day.

"Will it make a major difference in the campaign? I'm not sure it will," said Stephen Weissman, associate director for policy at the nonpartisan Campaign Finance Institute, of Obama's private money advantage.

To be sure, Obama has unveiled a strategy that could stretch GOP resources thin. If he follows through on plans to advertise heavily in traditional GOP strongholds in the South and the Midwest, he could force McCain to respond in kind. President Bush's record-low approval ratings continue to hamper GOP fundraising efforts. And the RNC faces added pressure to shore up the struggling National Republican Congressional Committee.

Obama "is going to be able to campaign in more states, put more staff on the ground, extend the electoral college playing field and advertise earlier and more often, especially in the crucial battleground states," said Tony Corrado, professor of government at Colby College in Waterville, Maine. Obama's extraordinary network of low-dollar campaign donors has defined the race, according to Corrado.

Still, there are signs that Obama's much-vaunted political money machine may be slowing down. His individual contributions of $21.1 million in May continued a three-month downward trend, according to CFI, contrasting with a high of $54.1 million in February. Obama is also shifting to a more conventional fundraising style, reaching out to top donors and bundlers who helped Sen. Hillary Rodham Clinton, D-N.Y.

Obama's very success as a fundraiser carries risks of its own. The presumptive Democratic nominee now faces pressure to help Clinton offset her $22 million campaign debt, plus the more than $11 million deficit faced by the Denver host committee for the DNC's August convention. His deep pockets also may help galvanize GOP donors. A May RNC solicitation letter signed by McCain exhorted donors that the "Democrats and their liberal allies are building the most massive political fundraising machine ever seen."

Obama's decision to reverse course on public financing -- he'd pledged earlier to opt into the system if his GOP opponent did -- could also draw more bad press. Though opting out does not appear to have hurt Obama in the polls so far, it has given McCain another line of attack. And if Obama's massive war chest becomes increasingly reliant on big donors and bundlers, it could disillusion youth voters.

"Playing the campaign finance game the old-fashioned way... most likely will fatten his coffers, but also will increase the attention paid to the potential hypocrisy there," observed University of Texas professor of government Bruce Buchanan. "That could alienate those idealistic young people."

Election after election, political scientists note, has demonstrated that money isn't everything. Just ask Obama -- he outspent Clinton in several key primary states where he ultimately lost. Obama's financial advantage will help introduce him to voters and force McCain and his allies to work harder. But it's only one of many factors in play.

"It isn't just the money; it's whether your message resonates," said Weissman. "And I think the truth in this election is that both candidates, regardless of what the ratio is at this point, have ample resources to get their message out."