Hillary Clinton is seeking to slow rival Barack Obama's momentum ahead of three more contests in the race for the Democratic nomination for US president.
Polls give Mr Obama a slight lead going into bi-party primaries in Maryland, Virginia and the District of Columbia.
Mr Obama won in Washington state, Louisiana, Nebraska, Maine and the US Virgin Islands at the weekend, keeping him neck-and-neck with Mrs Clinton.
She appointed a new campaign manager after the weekend's setbacks.
12 states, 1,136 delegates
Arizona, Arkansas, California, Florida, Massachusetts, Michigan, Nevada, New Hampshire, New Jersey, New York, Oklahoma, Tennessee
19 states, 1,108 delegates
Alabama, Alaska, Colorado, Connecticut, Delaware, Georgia, Idaho, Illinois, Iowa, Kansas, Louisiana, Maine, Minnesota, Missouri, Nebraska, North Dakota, South Carolina, Utah, Washington state
2,025 delegates needed for nomination. Source AP (includes all kinds of delegates)
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8 states, 234 delegates
Alabama, Arkansas, Georgia, Iowa, Tennessee, West Virginia, Kansas, Louisiana
12 states, 719 delegates
New Hampshire, Arizona, California, Connecticut, Delaware, Florida, Illinois, Missouri, New Jersey, New York, Oklahoma, South Carolina, Washington state
11 states, 282 delegates
Alaska, Colorado, Maine, Massachusetts, Michigan, Minnesota, Montana, Nevada, North Dakota, Wyoming, Utah
1,191 delegates needed for nomination. Source: AP (includes all kinds of delegates) Maggie Williams, who was the New York senator's chief of staff when her husband was serving as US president, will take over from Patti Solis Doyle, who has decided to step down.
Speaking to Chicago television reporters on Monday, Mrs Clinton sought to play down the importance of the move and said Ms Doyle would be staying on as an adviser.
"There really is not a significant change; we've really just got to get more help," Mrs Clinton said.
She and Mr Obama face a long, drawn-out battle after neither was able to deliver a knockout blow in the 22 state contests of Super Tuesday on 5 February.
Each is about half way to winning the 2,025 delegates needed to secure victory at the Democratic Party's national convention in August.
Both candidates have been campaigning hard ahead of Tuesday's so-called Potomac Primary, named after the river that runs through the two states and the nation's capital.
Virginia has 83 delegates up for grabs, while Maryland offers 70 and the District of Columbia has 15.
On the Republican side, front-runner John McCain will be hoping for a strong result from the Potomac Primary, where polls suggest he has the edge.
Baltimore voters give their views ahead of primary elections in Maryland
Although he won primary polling in Washington state on Saturday, correspondents say the Arizona senator still has some work to do to unite his party, as Mike Huckabee won in Kansas and Louisiana.
Mr McCain faces continuing criticisms from leading party members who have questioned his conservative credentials.
However, he picked up the endorsement of evangelical leader and anti-abortion campaigner Gary Bauer on Monday, which may raise his stock with Christian evangelical voters.
It came a day after President George W Bush described him as a "true conservative" in a taped interview.
Tuesday: Maryland, Virginia and Washington DC (bi-party)
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Mr Huckabee and third-placed Ron Paul have been coming under pressure to step aside for the sake of party unity, but the former said on Saturday he had no intention of pulling out.
Mr McCain has a wide lead with 719 delegates to Mr Huckabee's 234 and Mr Paul's 14.
Mitt Romney, who suspended his campaign last week, still has 298 delegates.