There are strong indications that Barack Obama is close to clinching the Democratic Party's presidential nomination.
Polling in the final two primaries in South Dakota and Montana ends at 2am and 3am (Irish time) respectively, with results expected shortly after.
Projections from Associated Press indicate that Mr Obama will become the first black candidate to lead a major US party into a campaign for the White House.
Hillary Clinton today said she was open to the possibility of being his vice presidential running mate.
The former first lady made the comment in a conference call with fellow members of New York's congressional delegation.
Her campaign earlier firmly denied US media reports that she planned to concede the Democratic presidential race to Barack Obama in a speech in New York tonight.
It has also been confirmed that former President Jimmy Carter is to endorse Senator Barack Obama as the Democratic nominee for president when tonight's polls close.
Mr Carter, a superdelegate, has remained neutral in the nomination race but has hinted his support for the Illinois senator.
Democrats in the two states cast the final votes in a gruelling battle for the right to face Republican John McCain in November's presidential election, with 31 delegates to the August convention in Denver at stake.
Mr Obama is within about 15 delegates of the 2,118 he needs to capture the nomination.
He could hit that number as soon as tonight depending on how quickly he wins commitments from nearly 200 uncommitted superdelegates - party officials who are free to back any candidate.
'There are a lot of superdelegates who are waiting for the last couple of contests but I think that they are going to be making decisions fairly quickly after that,' Mr Obama told reporters in Michigan yesterday.
US Representative James Clyburn of South Carolina, the third-ranking Democrat in the US House of Representatives and the top-ranking black member of Congress, backed Mr Obama this morning.
'Today the process ends,' Mr Clyburn told NBC's 'Today' programme. Other House members were expected to follow his lead, party sources said.
Senator Clinton and her campaign sent mixed signals yesterday about how long she would stay in a presidential race that she began as a heavy favourite but now has little chance of winning.
Campaigning in South Dakota, Ms Clinton said the end of the voting marked 'the beginning of a new phase of the campaign' in which she will plead her case to superdelegates that she would be the strongest candidate against Mr McCain in November.
'The decision will fall to the delegates empowered to vote at the Democratic convention. I will be spending the coming days making my case to those delegates,' she told supporters in Yankton, South Dakota.
But her husband, former President Bill Clinton, sounded like he was counting down the hours at a campaign stop in South Dakota on Monday.
'This may be the last day I'm ever involved in a campaign of this kind,' he said.
Clinton pitch to superdelegates
One of Senator Clinton's top supporters, Governor Ed Rendell of Pennsylvania, told CNN he expected her to spend Wednesday and Thursday making her pitch to superdelegates.
'I don't think it's going to prevail, to be candid,' he said. 'Senator Obama is going to get the delegates he needs certainly by the end of the week, then I think Senator Clinton is going to do the right thing and move fairly decisively to unify the party and we will all follow her lead.'
With no more campaign trips to plan, workers who handle Senator Clinton's advance travel arrangements have been told to go to New York or head home until further notice, aides said.
Mr Obama plans a victory celebration to kick-off the general-election campaign against Mr McCain after the South Dakota and Montana polls close tonight.
The Obama campaign has been urging superdelegates to make their endorsement before the voting ends on Tuesday, so the delegates he wins in the two states can put him over the top in the Democratic race.
A group of 17 uncommitted Senate Democrats met on Monday to discuss a potential endorsement of Mr Obama. Many are poised to announce either today or wait another day to give Senator Clinton a chance to bow out, Senate aides said.