WASHINGTON — Barack Obama was the first presidential candidate to raise more than $100 million in a month and in 2008 was the first to forgo public money for his campaign. Now, he faces the very real threat of being the first president to be outspent by a challenger.

Obama, who four years ago broke just about every fundraising record for a presidential hopeful, has now been forced to look his supporters in the eye and confess he might not keep pace with Republican Mitt Romney. It’s a sobering realization for his campaign, which had imagined an unlimited budget for ads, offices and mail.

“I will be the first president in modern history to be outspent in his re-election campaign,” Obama wrote to supporters recently.

It wasn’t supposed to be this way. Conservatives just two years ago feared Obama would raise and spend a billion dollars in the 2012 campaign. Now, there is a real possibility that Romney and his official partners at the Republican National Committee could overtake Obama in total spending.

How did Obama go from fundraising juggernaut to money chaser in just four years?

In the early days of the 2007 primaries, he used fundraising success to puncture Hillary Rodham Clinton’s aura of inevitability. Obama surpassed Clinton’s primary fundraising in the first two quarters of that year — $25 million to Clinton’s $20 million from January to April, and $31 million to Clinton’s $21 million in the three months that followed.

The numbers shocked observers and inspired supporters to give even more to the fresh-faced, first-term senator from Illinois. But now that magic seems elusive.

“They bought into hope and change and they’re not getting it. There’s some buyers’ remorse,” said Greg Mueller, a Republican strategist who is a veteran of Pat Buchanan’s presidential campaigns.

Then, the potential was so great that Obama became the first modern candidate to bypass the public financing available to presidential candidates, and the spending limits that come with it, since the system was created in 1976 in the wake of the Watergate scandal.

At the same time, Obama shunned independent groups that sought to help his campaign and told supporters not to give to them. In his mind, he simply didn’t need them and urged allies to shut down independent efforts to attack rival John McCain. He preferred to level criticism of his choosing, on his own terms.

But two years later, midterm elections yielded defeats for Democrats who lost their majority in the House. Early fundraising reports in 2011 showed the Republican independent groups were awash in cash, and Obama relented. With an economy that hasn’t recovered quickly enough for voters, he opted to accept whatever help he can find, giving the go-ahead for outside groups to raise and spend cash on his behalf. His top advisers now are helping the groups he once abhorred, but he sounds unhappy about it.

“In the next four months … there’s going to be more money spent than we’ve ever seen before. Folks writing $10 million checks to try to beat me, running ads with scary voices,” Obama lamented at a fundraiser Tuesday in Texas.

Part of the about-face was fueled by the Republican primaries. Casino magnate Sheldon Adelson donated $20 million to an independent group that, for a time, kept former House Speaker Newt Gingrich afloat. Adelson now is backing a pro-Romney group with at least another $10 million.

Like Obama’s official campaign and its partners at the Democratic National Committee, outside groups on the Democratic side are at an admitted disadvantage.

“There’s no doubt that Romney’s campaign and the super PACs supporting him will outspend the president’s campaign and the super PACs on our side,” said Bill Burton, a former Obama aide who is now running an independent pro-Obama group. “There’s more money on the Republican side.”

Obama demonized Wall Street bankers and they responded by closing their wallets. He also has called on wealthier Americans to pay more in taxes — hardly an inspiration to donate, his advisers concede. For some of his most liberal supporters, he has not done enough to promote stronger unions or tougher environmental laws.

And, unlike four years ago, Obama is not campaigning as an optimistic vessel of hope and change.

Obama and his allied DNC committees raised $71 million in June, short of Romney’s and Republicans’ $106 million. Romney’s June haul was just the second time in history that an American campaign and its partner committees passed the $100 million mark, and signals the 2012 GOP presidential fundraising could break Obama’s 2008 record of $745 million. The reports also mark a second consecutive month Obama trailed his rival.

“We had our best fundraising month yet, and we still fell about $35 million short,” campaign chief operating officer Ann Marie Habershaw told supporters in an email that asked for as little as $3 to help.

That’s not to say Obama is broke or even certain to be outspent. And if he is, it’s unlikely to become a determining factor in the election. While campaigns need money to pay staff, finance travel and buy television ads, money alone does not win elections when both candidates are financially competitive.

From the days when Obama and Romney formally announced their campaigns, Obama and his affiliated party groups have raised $552.5 million, compared with Romney’s $394.9 million. The nonpartisan Sunlight Foundation broke down the numbers and noted that Romney would need to bring in $39.5 million more than Obama each month to exceed his total.

That leaves a steep climb for Romney, but not an impossible one. Conservatives who were skeptical of Romney now are rallying behind the GOP nominee after a topsy-turvy primary season that saw their favored candidates come up short. Polling shows Republicans eager to vote Obama out of office.

Romney’s vice presidential selection in the coming weeks will create additional buzz and likely unleash a fundraising wave for the final months of the campaign.

Never before has an incumbent president failed to outraise a challenger, according to the Center for Responsive Politics, a campaign finance watchdog. In Obama’s record-setting 2008 campaign, he made history in September by raising $150 million.

Now, it’s Romney’s turn to try to shatter that record — and for Obama to defend his.