Before vote counting began, Obama had 1,911 delegates, little more than 100 shy of the 2,026 needed to become the first black presidential nominee of a major party, in the latest CBS News count. The former first lady had 1,715.
Regardless of the results of the night's two primaries, Obama decided to mark a victory of sorts. He arranged an evening appearance in Iowa, site of his critical Jan. 3 caucus triumph, to claim a majority of the delegates at stake in all 56 contests on the campaign calendar.
Clinton looked for a consolation for the strongest presidential campaign of any woman in history. She hoped to finish with more votes than her rival in all the contests combined, including Florida and Michigan, which were striped of their delegates by the national party for moving up their primary dates.
Campaigning with his wife in Kentucky, former President Clinton dismissed Obama's inevitable claim on pledged delegates.
"There won't be tonight, unless you decapitate Michigan and Florida, which violates our values and is dumb politics," Bill Clinton said.
Kentucky, where Hillary Clinton concentrated much of her efforts in recent days, had 51 convention delegates at stake.
Clinton and her husband spent more than an hour Tuesday morning working the room at a diner in Louisville. They signed autographs, cuddled a baby and shook hands with diners, some of whom told the former first lady they had already voted for her.
"I'm going to work hard for you," she replied to one woman who volunteered she'd voted for Clinton.
Polls opened in Kentucky at 6 a.m. EDT and elections officials reported few problems.
Oregon, where Obama invested his time and drew a crowd estimated by police at 75,000 over the weekend, had 52 delegates at stake. The state also had the distinction of staging the only contest without a designated polling day. Instead, under a vote-by-mail system, election officials tallied all ballots received by 11 p.m. on primary day.
Obama was favored in Oregon, where supporters delivered the largest crowd of his campaign on Sunday.
The only primaries remaining are Puerto Rico, on June 1, followed two days later by South Dakota and Montana.
Increasingly, Obama has been concentrating his campaign on, the Republican nominee-in-waiting, rather than on Clinton.
The former first lady has jettisoned the sharp attacks against Obama that characterized the race only a few weeks ago, although she bristled on Monday at his decision to focus on the fall campaign. "You can declare yourself anything, but if you don't have the votes, it doesn't matter," she said in a satellite interview with an Oregon television station.
Even so, there was no shortage of signs that the closest Democratic nominating campaign in a generation was reaching its final stages after drawing more than 33 million voters to the polls and shattering numerous turnout records along the way.
Former Sen. Tom Daschle, a key Obama adviser, said now is the time for Democrats to coalesce behind Obama in order to defeat Republican nominee-in-waiting.
"We want to begin the process of bringing this party together, and I think that over the last few weeks we've seen indications at virtually all levels in both campaigns that there's a desire to do that," Daschle told CBS News' "The Early Show" Tuesday morning. "That doesn't mean we're going to do it tomorrow or the next day, but clearly there is a desire to unify. We know that the differences between Barack Obama and Hillary Clinton pale by comparison [to] the differences that we now see between Barack Obama and John McCain."
As recently as May 6, Obama trailed Clinton among superdelegates, officeholders and party leaders who will attend the national convention by virtue of their positions.
But in the days following his convincing victory in the North Carolina primary and his narrow defeat in Indiana, Obama has gained the support of dozens of superdelegates and taken the lead in that category. Clinton has gained far fewer over that period.
Obama also has picked up the endorsement of former Sen. John Edwards, who dropped out of the race in the early going; two labor unions, and NARAL Pro-Choice America. The abortion rights advocacy organization has supported Clinton throughout her political career.
Fundraisers for the two campaigns have held quiet discussions on working together in the fall campaign.
Additionally, Obama's top strategist, David Axelrod, disclosed he had contacted Clinton's former campaign manager about joining forces for the general election. Patti Solis Doyle confirmed what she called informal conversations about how she might help the Illinois senator if, as expected, he secures the presidential nomination.