The "Potomac Primary" also put attention on a part of the country not used to playing a crucial role in the presidential nominating process.
"At least we feel like we have a say in it this time," said Michael Dowling, a 57-year-old Annapolis resident who voted early Tuesday and couldn't recall the last time he felt the Maryland primary had a role in choosing the nominee.
Early turnout in Virginia was reported high and city officials in the District of Columbia were hoping that a swath of new registered voters would show up at the polls. Maryland election officials were also projecting a strong turnout, particularly in the Democratic race.
With 168 delegates at stake in the Democratic contests, Obama hoped to expand the lead in the delegate count he won after convincing weekend victories in Louisiana, Nebraska, Washington state, the Virgin Islands and Maine.
The contests also come days after Clinton announced she was replacing her campaign manager. Aides to the former first lady concede she is in the midst of a difficult period in which she could lose 10 straight contests. She is hoping to rebound on March 4, in primaries in Ohio and Texas, states where both candidates have already begun television advertising.
A source close to the campaign is downplaying expectations, telling CBS News' Fernando Suarez that "we will lose all three (contests) today, probably by wide margins."
"We are still actively working them because we feel like there are opportunities to pick up delegates," the source said, pointing out that since delegates are allocated proportionally, she can still pick up delegates even if she loses statewide.
"There are several districts in Virginia, for example, that we have targeted because they have odd number delegates and we think we have some opportunities to pick up the one," the source said. "So, while we expect to lose we're focused on the delegate math." (Read more in the From The Road blog.)
According to delegate estimates compiled by CBS News Obama has 1,143 delegates while Clinton has 1,132. A candidate needs 2,025 delegates to secure the nomination. (View the Democratic delegate scorecard.)
Obama has campaigned before huge crowds in recent days, and far outspent his rival on television advertising in the states participating in the regional primary in Maryland, Virginia and the District of Columbia.
He began airing commercials in the region more than a week ago, and spent an estimated $1.4 million. Clinton began hers last Friday, at a cost estimated at $210,000.
With Clinton facing a series of possible defeats, and Obama riding a wave of momentum, the two camps debated which contender is more likely to defeat McCain in the general election.
An Associated Press-Ipsos poll found Obama with a narrow lead over the Arizona senator in a potential match-up, and Clinton running about even.
"We bring in voters who haven't given Democrats a chance" in the past, said Obama pollster Cornell Belcher, citing support from independents.
Mark Penn, Clinton's chief strategist, countered that she holds appeal for women voters and Hispanics. "Hillary Clinton has a coalition of voters well-suited to winning the general election," he said.
While it expects victories in all three races tonight, the Obama campaign is already trying to keep expectations about any momentum gained in check, particularly regarding the Feb. 19 primary in Wisconsin.
"I don't think that it's about momentum, it's about reality and the math," Obama campaign manager David Plouffe said on a conference call with reporters, according to CBS News' Maria Gavrilovic. )(Read more in From The Road.)
Plouffe and Gov. Jim Doyle, D-Wisc., disputed the notion that Obama has momentum from his slew of recent wins. Doyle said voters in Wisconsin are evaluating both Clinton and Obama, and are not affected by previous outcomes.
Plouffe described Wisconsin has a battleground state, and said the campaign is aggressively organizing in the state. He believes that the timing of the primary will give Obama an edge in Wisconsin because he has more time to campaign.
Among Republicans, McCain, the faraway front-runner, hoped to rebound from a poor weekend showing. There were 116 GOP delegates at stake.
McCain lost caucuses in Kansas and a primary in Louisiana on Saturday to former Arkansas Gov., his last remaining major rival. When asked by "Up to the Minute" Anchor Meg Oliver if he planned to stay in the race to secure a cabinet position, Huckabee replied, "There's less likelyhood of me accepting a cabinet position than there would be of me becoming Hillary Clinton's running mate. So let me take that off the table. I told some group today - they thought I was hanging in so I could run for the Senate - and I told them I'd rather tattoo my body and go on a tour with Amy Winehouse than... run for the senate." McCain won caucuses in Washington state.
CBS News delegate estimates McCain has won 705 delegates so far, giving him nearly 60 percent of the 1,191 required to secure the nomination. Huckabee, a former Arkansas governor, is far behind, with 199 delegates. (View the Republican delegate scorecard).
It takes 1,191 delegates to clinch the nomination, and McCain appears to be on track to reach the target by late April.
"We have close to 800 delegates. Last time I checked, Governor Huckabee had very few, so I think I'm happy with the situation I'm in," McCain said Monday. "I'm quite pleased, recognizing that we have a lot of work to do."
In Annapolis, McCain chuckled at a question about why people persist in voting for Huckabee despite McCain's lock on the nomination.
"Because they like him," McCain said. "I never expected a unanimous vote, although I would certainly like to have that."