"Fifteen months ago, in the depths of winter, it was in this great state where we took the first steps of an unlikely journey to change America," he said at an outdoor rally bookended by news of a landslide loss in Kentucky and a convincing victory in Oregon. "Tonight, in the fullness of spring, with the help of those who stood up from Portland to Louisville, we have returned to Iowa with a majority of delegates elected by the American people, and you have put us within reach of the Democratic nomination for president of the United States."
Obama's lush rhetoric was encouraged by a seemingly inexorable arithmetic that moved him close to the finish line. He entered the evening needing just 17 delegates to capture a majority of the 3,253 pledged delegates--a beachhead that is almost impossible for Clinton to overcome.
After passing that threshold Tuesday, the Illinois senator now finds himself within eyesight of the magic number of 2,025, the number of delegates and superdelegates needed to clinch the nomination under the rules set by the Democratic National Committee.
The Clinton campaign disputes that figure and instead insists on using 2,210 as the key metric--a number predicated on the counting of delegates from Michigan and Florida--but Obama's victory Tuesday left her at the mercy of the DNC's Rules and Bylaws Committee, which meets May 31 to determine appeals from the two wayward states.
Obama chose Des Moines for his speech both because it's where his bid ignited and because he thinks he can add Iowa - which George W. Bush won in 2004 - to the Democratic ledger.
"It is appropriate that we go back to Iowa where this all began and really launched this campaign to note that," Obama strategist David Axelrod told reporters aboard the campaign plane earlier in the day. "I don't think anyone has won the majority of pledged delegates and not been the nominee of the party so it is obviously very important."
Yet Obama's broad strokes bumped up Tuesday evening against the narrow boundaries of the map on which presidential elections are actually fought. He delivered his remarks after taking a shellacking in Kentucky, where he lost by a roughly two-to-one margin. He failed to pick up a single delegate in one Eastern Kentucky congressional district, underscoring questions about his ability to compete in the Appalachian East.
Axelrod said Clinton's strong performance in Kentucky was "not unexpected."
"This was always a strong state for her and she has been hunkered down essentially for the last week," he said. "We made a different choice. We had other imperatives. While we competed in Kentucky, we were obviously competing in Oregon, where we did very, very well and we were trying to touch base with some of these swing states in the general election. She was rewarded in part for spending all of her time in that one state."
At the same time his campaign was downplaying Clinton's victory, Obama Tuesday began offering olive branches to Clinton. In an interview with ABC News, he agreed with her complaints that she has faced sexism on the campaign trail. In his Des Moines remarks, he complimented her as "one of the most formidable candidates to ever run for this office."
"In her 35 years of public service, Senator Hillary Rodham Clinton has never given up on her fight for the American people," he said. "We have had our disagreements during this campaign, but we all admire her courage, her commitment and her perseverance. No matter how this primary ends, Senator Clinton has shattered myths and broken barriers and changed the America in which my daughters and yours will come of age."
It's not Clinton herself that Obama needs to charm, though. It's her voters, some of whom - exit poll have suggested - are motivated as much by alienation from him as by a fondness for her. In Kentucky, 55 percent said they would not be satisfied if Obama won the nomination. Forty-eight percent said they would be satisfied only if Clinton wins.
While Clinton was careful Tuesday to stress the need for party unity, she nevertheless signaled her intention to see the campaign to its June 3 finish, when Montana and South Dakota are the last states to vote.
"Tonight we have achieved an important victory," she said in her victory speech. "We're winning the popular vote and I'm more determined than ever to see that every vote is cast and every ballot counted."
"I'm told that more people have voted for me than for anyone else who has run for the Democratic nomination," she said.
Still, as Obama's speech highlighted, the delegate count left him on the brink of nomination. Clinton's Kentucky victory didn't reveal anything that wasn't already known to the Democratic superdelegates who will determine the fate of her candidacy. And by breaking the threshold of a majority of pledged delegates, Obama made it even more difficult for Clinton to win over uncommitted superdelegates since they are reluctant to overturn the will of the voters.
That left him on the same glide path to victory, waylaid only slightly after a second consecutive week featuring a crushing primary defeat.
On Clinton's plane en route from Louisville to Washington, campaign chairman Terry McAuliffe predicted the campaign's second landslide win in as many weeks would at least keep undecided superdelegates on the fence.
"Most of them want to wait until June 3," he said. "In fairness, you have West Virginia, which was a great win last week. I think Kentucky will give them pause now. She has won two weeks in a row. I think tonight was a big deal."
Carrie Budoff Brown and Kenneth P. Vogel contributed to this report.