Barack Obama stands poised to claim his place in history as the first black presidential nominee of a major U.S. party after the final pair of primaries bring to a close the marathon Democratic presidential race.
The grueling nomination battle has shattered fundraising and voter turnout records while stoking worries that the prolonged battle between Obama and Hillary Rodham Clinton could undermine the Democratic Party's chances of securing the White House.
The former first lady pressed on with her campaign on Monday, but there were increasing signals that she was preparing to acknowledge defeat in her own groundbreaking bid to become the first female U.S. president.
Obama arranged a Tuesday night speech to launch his general election campaign in the same arena in St. Paul, Minnesota, where the Republican National Convention will nominate John McCain in September.
Obama was just 41.5 delegates shy of the 2,118, needed to gain the nomination at the Democratic party convention in Denver. He was expected to overcome that hurdle soon after Tuesday's final primaries in South Dakota and Montana by winning over more superdelegates, the party leaders and elected officials who may choose whomever they like at the August convention.
He gained 5.5 superdelegates during the day on Monday, including Rep. Jim Clyburn of South Carolina, a member of the House leadership who scheduled a formal announcement for Tuesday.
Obama's aides prodded uncommitted lawmakers and other superdelegates to climb on board quickly - as Clinton struggled to hold back the tide and some of her supporters hinted that the end of her campaign was near.
Acknowledging that he must woo her passionate supporters, Obama said he has asked Clinton for a meeting on her terms. "I told her that once the dust settled I'm looking forward to meeting with her at a time and place of her choosing," he said. He was disclosing the contents of a conversation the two rivals had on Sunday night but did not describe her response.
"The sooner we can bring the party together, the better, so we can focus on John McCain and taking back the White House," he told reporters.
He also said he would begin thinking about a vice presidential running mate "the day after I have gotten that last delegate needed to officially claim the nomination."
There were signs that despite her promise to fight on, Clinton was preparing to acknowledge defeat once Obama gained the final delegates needed.
"This may be the last day I'm ever involved in a campaign of this kind," former President Bill Clinton said in South Dakota.
The former first lady campaigned into the night in the state, scratching for an upset primary triumph that could somehow persuade uncommitted superdelegates to back her, before heading home to New York for a post-primary appearance with supporters on Tuesday night - a rare departure from her campaign practice
"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 dismissed fears that the party would be unable to unite for the fall campaign. "Senator Clinton has run an outstanding race, she is an outstanding public servant and she and I will be working on November," he said.
Democratic Party leaders watched from the sidelines, eager for a quick end to a race that drew record millions to voting booths but also exposed racial, gender and other divisions within the party.
Officials said that if Obama failed to gain 2,118 delegates by Tuesday night, one possibility under discussion was for Senate Majority Leader Harry Reid and Speaker Nancy Pelosi to issue a statement on Wednesday urging superdelegates to state their preferences as soon as possible.
Clinton has had a strong run through the late primaries, including a lopsided victory on Sunday in the Puerto Rico primary, and she has repeatedly declined to say she would concede defeat if her rival appeared to gain the delegates he needs.
She entered the week with an insurgent strategy not only to win over undecided superdelegates but to peel away Obama's support from those who already have committed to back him for the nomination.
But, a top aide to Clinton, Harold Ickes, conceded that Obama was likely to reach the delegate threshold by Wednesday, and that Clinton would need some time to consider her next step.
All told, Republican and Democratic candidates have spent almost $195 million (€126 million) on television ads so far in this extended primary season, with the Democratic contenders paying the bulk at about $136 million (€87.6 million), an analysis of political advertising shows. Obama outspent all the Republicans combined by more than $17 million ($NZ172.6 million).
Obama wants to formally kick off his general election campaign against McCain in a victory speech on Tuesday night as the final primary polls close.
"Senator Obama is trying to line up people that are going to come out for him tomorrow during the day so that he'll have enough that puts him over the top that he can declare victory tomorrow," said Pennsylvania Rep. Jason Altmire, one of about 200 uncommitted superdelegates under pressure to take a side in the contest.
Obama is favored in both South Dakota and Montana, which offer a combined total of 31 delegates. He currently has 2,076.5 delegates, to Clinton's 1,917.5.
The Illinois senator has made up most of the ground he lost on Saturday when the national party's rules committee agreed to reinstate delegates from Michigan and Florida, but with only half a vote each instead of a full vote at the convention. The party had initially refused to seat the delegates as punishment for scheduling their contests in violation of party rules.
Meanwhile, McCain will mark the start of the general election campaign during a speech on Tuesday evening in the New Orleans suburb of Kenner, Louisiana, a searing symbol of government inaction after Hurricane Katrina.
The Republican nominee-in-waiting plans to draw contrasts with Obama on a range of issues and argue that the Democrat offers the wrong kind of change while he offers the right kind.
Previewing his remarks, McCain told reporters on his campaign bus in Nashville, Tennessee: "The message is change. It's real change. I think it's clear I have a record of working across the aisle. Senator Obama does not. I think it's my record of reform and efforts to change the way we do business in Washington. He has the most liberal voting record of any senator in the U.S. Senate."
