Barack Obama and Hillary Clinton end their historic Democratic presidential battle on Tuesday with two nominating contests that could help Obama clinch the nomination and push Clinton from the race.
Democrats in South Dakota and Montana 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.
Obama is within about 40 delegates of the 2,118 he needs to capture the nomination and become the first black nominee of a major U.S. political party.
He could hit that number as soon as Tuesday night 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," Obama told reporters in Michigan on Monday.
Voting ends in South Dakota at 7 p.m. MDT/9 p.m. EDT (2 a.m. British time), and in Montana an hour later, with results expected shortly after.
Clinton and her campaign sent mixed signals on Monday 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, Clinton said the end of the voting on Tuesday 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 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.
