"People are watching this race and they're wondering ... I win, he wins, I win, he wins ... it's so close. That says a lot about how passionate our supporters are ... but I can assure you that no matter what happens I will work for the nominee of the Democratic party," she said.
Clinton continued to wage a strong campaign in the close weeks, though attacks on her opponent all but disappeared from her speeches and events.
Instead, Clinton focused on pushing the Democratic National Committee to recognize the disputed votes in Florida and Michigan and racked up impressive, double digit wins in West Virginia, Kentucky, and Puerto Rico.
But even big wins could not sustain Clinton.
Clinton did succeed in getting some delegates recognized from Michigan and Florida but not all that she wanted.
The Democratic National Committee's rules and bylaws committee voted to seat the full Michigan and Florida delegations, but with each delegate getting only half a vote. In the arrangement, Clinton picked up 94.5 total delegates to Obama's 65.5, for a net gain of 19 delegates overall.
Clinton supporters angry, some disrupting the committee proceedings, chanting, "Denver! Denver! Denver!" as a sign they want Clinton to take her fight all the way to the Democratic Convention.
"Mrs. Clinton has instructed me to reserve her right to take this to the Credentials Committee," Harold Ickes, a key Clinton strategist told the Democratic National Committee.
In the end, Clinton faced one insurmountable reality: math.
Obama emerged from one of the longest Democratic contests in recent history ahead in pledged delegates, though still officially short of the number necessary to clinch the nomination.
The only option left available to Clinton was to push her fight to the Democratic convention in late August.
Such a protracted fight would have undoubtedly left the Democratic nominee damaged, forced to face a Republican nominee free to campaign through the summer against a disorganized and divided Democratic party.
Instead Clinton will end her bid, allowing Democrats to join ranks around Obama and head into the next stage of the election against McCain.
Obama will officially accept the Democratic nomination on Aug. 28, 2008, in Denver, coincidentally on the 45th anniversary of the Rev. Martin Luther King, Jr.'s "I Have a Dream" speech -- another memorable moment in a campaign already making history.
ABC News' Rick Klein, Jennifer Parker, and Jake Tapper contributed to this report.