Caucuses are being held in the US state of Wyoming on Saturday to choose 12 delegates to the Democratic Party nomination convention in August.
Senator Barack Obama is trying to retake the momentum he lost to Hillary Clinton in last Tuesday's vote.
Mr Obama still leads the delegate count.
For the Republicans, John McCain has secured the party's nomination.
Mr Obama heads Fears for 100 in Congo sinking ...
Obama takes Bill Clinton to task ... into the caucuses amid controversy caused by one of his aides who called Mrs Clinton "a monster".
Samantha Power - a Harvard professor who has advised Mr Obama on foreign policy - resigned on Friday.
She had told the Scotsman newspaper: "She [Hillary Clinton] is a monster, too - that is off the record - she is stooping to anything."
Ms Power has since apologised.
Iraq exit strategy
Caucuses are being held at 23 electoral districts in Wyoming, beginning at 1600 GMT. From there the candidates head for Mississippi, which is holding primaries on Tuesday.
Mr Obama told supporters in Wyoming on Friday that he would end the war in Iraq in 2009 - another issue causing a stir in his camp after another remark by his former foreign policy adviser.
Samantha Power told the BBC earlier this week that the Illinois senator's position that he would withdraw all troops within 16 months was a "best-case scenario" that he would revisit if he became president.
Campaigning in Mississippi, Mrs Clinton also called for an end to the Iraq involvement.
"He [Mr Obama] has attacked me continuously for having no hard exit date, and now we learn he doesn't have one, in fact he doesn't have a plan at all," she said.
Mr Obama currently has 1,567 delegates against Mrs Clinton's 1,462. It takes 2,025 to secure the party's nomination.
Both candidates have reported massive fund-raising totals for February, with Mr Obama bringing in $55m (Ј28m) and Mrs Clinton $35m (Ј17m).
After Tuesday's primary in Mississippi, in which 33 Democratic delegates will be awarded, the next major battle will be the Pennsylvania primary on 22 April, with 158 delegates up for grabs.
Debate on whether to hold fresh ballots in Florida and Michigan also continues.
Both states were told their delegates would not be seated at the party's August national convention - meaning they cannot vote on who should be the Democratic presidential candidate - after they breached party rules by holding primary elections before 5 February.
Aides to Mrs Clinton have indicated they would be open to new elections being held, saying they believe her prospects would be good.
(BBC)
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