Maine Democrats are holding caucuses to decide between Hillary Clinton and Barack Obama, as the process to choose a presidential candidate continues.
The Clinton campaign says she expects to win in Maine
It follows a weekend of gains by Mr Obama in the battle for the Democratic Party's presidential nomination.
The Illinois senator won in polls Louisiana, Nebraska and Washington state and the US Virgin Islands.
Meanwhile, Mrs Clinton has appointed a new campaign manager a day after this weekend's setbacks.
Maggie Williams, who was the New York senator's chief of staff when her husband was serving as US president, will take over from Patty Doyle, who has decided to step down.
Campaign aides said Ms Doyle's decision was not urged upon her by Mrs Clinton or any senior members of her team, the Associated Press reports.
The change comes as Mr Obama's gains put him almost neck-and-neck with Mrs Clinton.
With every vote now counting in the race for the White House, the pair both made appearances in Maine on Saturday in an effort to boost support.
Correspondents are predicting a large voter turnout, despite a weather forecast of severe cold and heavy snow.
Following the Maine caucus, Mr Obama and Mrs Clinton head into a series of contests on Tuesday: Virginia, Maryland and the District of Columbia.
The BBC's James Coomarasamy, in Washington DC, says Mr Obama's easy victories in the Democratic contests on Saturday will not be decisive, but they will return him the initiative.
In a speech to Democrats in Virginia on Saturday, Mr Obama said the momentum of the results would be a lasting boost to his campaign.
"Today the voters from the West Coast to the Gulf coast, to the heart of America, stood up to say 'yes we can'... we won north, we won south, we won in between," he said.
But Mrs Clinton's campaign says that she expects to take victory at Sunday's contest in Maine and then in Texas and Ohio on 4 March.
For the Republicans, Mike Huckabee beat John McCain in Kansas and Louisiana, but Mr McCain won Washington state.
Our correspondent says that Mr Huckabee's victories in Kansas and Louisiana show that there is a socially conservative section of the Republican Party that is very suspicious of John McCain, so Mr McCain has some work to do to unite his party.
Mr Huckabee and third-placed Ron Paul have been coming under pressure to step aside for the sake of party unity.
Mr Huckabee said on Saturday that he had no intention of pulling out.
"Am I quitting? Let's get that settled right now. No, I'm not," he said.
"I majored in miracles, and I still believe in them."
Fight for funds
Going into the weekend's contests, Mr McCain had a wide lead with 719 delegates, to Mr Huckabee's 198 and Mr Paul's 14.
Mr Romney's suspended campaign still has 298 delegates.
RESULTS SO FAR
Hillary Clinton: 1095 delegates, 13 states
Barack Obama: 1070 delegates, 18 states
John McCain: 719 delegates, 12 states
Mike Huckabee: 234 delegates, 7 states
Ron Paul: 14 delegates, 0 states
On the Democratic side, Mr Obama and Mrs Clinton are facing the prospect of a long drawn-out battle after neither was able to deliver a knock-out blow in Super Tuesday's 22 state contests.
In advance of the weekend contests Mrs Clinton had won 1,055 delegates to Mr Obama's 998 of the 2,025 needed to secure victory at the Democratic party convention in August.
As well as fighting for every vote, the candidates are also jostling for funds. The Obama campaign says it has raised $7m (£3.6m) since Tuesday.
The Clinton camp is now claiming a similar fund raising bump, gaining $6.4m. They earlier admitted that the former first lady had lent her campaign $5m to paper over what aides called a "temporary cash flow problem".