The former first lady has given no hint of quitting the race, and she has said repeatedly she may continue her candidacy even beyond the end of the primaries.
But her husband, former President Clinton, strongly suggested otherwise. "This may be the last day I'm ever involved in a campaign of this kind," he said as he worked for his wife in South Dakota. That state, and Montana hold the final primaries of the campaign on Tuesday.
Obama, bidding to become the first black major party nominee in history, was 42.5 delegates shy of the 2,118, needed to clinch the nomination at the party's convention in Denver. He gained 4.5 during the day Monday, and one member of the House leadership, Rep. Jim Clyburn of South Carolina, readied an endorsement for Tuesday.
Obama's aides prodded uncommitted lawmakers and other "superdelegates" to climb on board quickly -- as Clinton struggled to hold back the tide.
Rep. Jason Altmire of Pennsylvania, who is uncommitted, said Obama's goal was to be in position to seal the nomination Tuesday night, once the votes are tallied from primaries in Montana and South Dakota. The first-term congressman, whose district voted for Clinton in the state's primary, said he would not be immediately joining the endorsers. "I'm not going to do anything before the results tomorrow night," he said.
Clinton, the long-ago front-runner, was not far behind Obama in delegates. She had 1917.5 after adding two during the day.
But there was no doubt that the historic nominating campaign, pitting a black man against a woman, was nearing an end.
If nothing else, the candidates' itineraries said as much.
The former first lady campaigned into the night in South Dakota, scratching for a primary triumph that could somehow persuade uncommitted superdelegates to back her, before heading home to New York for a post-primary appearance Tuesday night.
"I'm just very grateful we kept this campaign going until South Dakota would have the last word," she said at a restaurant in Rapid City.
Obama, confident of victory, looked ahead to the general election by campaigning in Michigan, a likely battleground state in the fall campaign.
He said that when he called Clinton on Sunday to congratulate her on her Puerto Rico primary victory, he broached the topic of a meeting.
"The sooner we can bring the party together, the sooner we can focus on John McCain and taking back the White House," he said.
Obama stopped short of a flat prediction that he would be able to claim victory Tuesday night when the delegates were allocated after the day's primaries. But he said, "It is my sense that between Tuesday and Wednesday we have a good chance of getting that number of delegates" needed for victory.