Barack Obama raised more than $265 million in 16 months. The question facing his presidential campaign is whether he can match or exceed that haul in the five months that remain before Election Day.

Now that his decision to bypass the limits of public financing is official, Obama’s efforts to join forces with fellow Democratic Sen. Hillary Rodham Clinton take on added significance. Her donors would be a rich vein to tap.

First, however, Clinton needs substantial help retiring her debt, now at more than $20 million. Many of her loyal donors have already contributed the maximum to her campaign, so she needs some new sources of money. That’s where Obama comes in _ his donors help her out, her donors help him.

“It’s far more productive for Obama to have Hillary 100 percent focused and engaged on campaigning and raising money for him in the fall rather than having to do fundraisers at the same time to retire her debt,” said Hassan Nemazee, a Clinton national finance chairman.

“It would clearly make life easier for those of us in the Clinton world who would like to help Senator Obama raise the types of moneys that are necessary from the Clinton world to be in a position to point out, ‘Look what Senator Obama has done for Senator Clinton.’”

Clinton and Obama will meet with her top fundraisers on Thursday in Washington, then both will campaign together on Friday.

Obama, who secured the Democratic Party nomination on June 3, enters the general election phase of the campaign with sound financial footing. But the all-but-nominated Republican, John McCain, is displaying improved fundraising and is boosted by a Republican National Committee that has reported more than 10 times more money in hand than its Democratic counterpart. McCain had his best fundraising month in May, collecting $21 million and socking more than $31 million in the bank.

Still, with his network of small donors and an infusion of new cash from Clinton contributors, Obama should have little trouble surpassing McCain’s fundraising in the fall. McCain has agreed to accept public money for the September-November fall campaign _ a step that limits him to $85 million in spending.

Some Obama backers point out that Obama’s vaunted Internet fundraising relied on major events to motivate small donors. Obama’s contributions tended to spike especially around primary elections. But the primaries are now over, summer vacations are ahead and donors may not be as driven to give between now and the Democratic National Convention in late August.

But Steve Weissman, associate director at the Campaign Finance Institute, pointed out that Democrat John Kerry in 2004 was able to raise $185 million between March, when he secured the nomination, and August, when the party held its convention.

“People are mobilized,” Weissman said. “This is the election.”

Obama, becoming the first major party candidate to operate outside the public financing system in the general election, said he is expecting McCain to have significant help from the Republican Party and from outside groups.

So far, though, few conservative outside groups have stepped into the presidential election and those that have have spent little money. Obama defended his decision in a news conference Friday.

“I don’t think I’m off the wall here to see that, you know, there are a lot of outside groups that are potentially going to be going after us hard,” he said. He also pointed out that McCain advisers have made a point of featuring the RNC’s financial advantage.

“So you know, this isn’t speculative on my part,” he said. “I think it’s something that we’ve seen in the past and it’s something that we continue to be concerned about.”

Still, even if Obama far out-raises McCain, that hardly guarantees an edge with voters. Obama outspent Clinton in several major primary battlegrounds, including Ohio and Pennsylvania, but lost anyway.

“You have to have more than money,” Weissman said. “If (McCain’s) message resonates, it won’t matter that Obama has more money.”