WASHINGTON (AP) — 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 withRepublican 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.