Meanwhile, Sen. Barack Obama, D-Ill., defeated Senator Hillary Clinton, D-N.Y., in the Virginia primary tonight, crushing what may have been Clinton's best chance at a Potomac primary win.
Washington, D.C. polls closed at 8:00pmET, and election officials in Maryland have extended voting statewide until 9:30pmET because of traffic delays caused by freezing temperatures, rain and sleet in some areas.
MCain, Obama Win Virginia
Clinton had hoped to perform strongly in Virginia's rural communities and among women and the state's sizeable Hispanic and immigrant population, but Obama ultimately prevailed in the state.
In a sign of further turmoil within Clinton's campaign, word came late tonight that the Clinton deputy campaign manager Mike Henry stepped down today, reports ABC News Kate Snow.
Henry is famous for a leaked memo he wrote suggesting that Clinton skip Iowa. He was loyal to former campaign manager Patti Solis Doyle, who was replaced two days ago.
Boding well for Obama, preliminary exit poll results suggest majorities of Democratic voters in Virginia and Maryland alike said the top attribute they're seeking in a candidate is the one who can "bring needed change" -- a message consistently promoted by Obama.
Obama won 88 percent of African American voters in Virginia and also split white voters with Clinton, with white voters favoring him over Clinton by a 12-point margin, according to preliminary exit results reported by ABC News' Gary Langer.
Obama was also helped by independents, who made up a fifth of voters in Virginia's open primary.
Going into today's primary in Maryland, McCain and Obama looked strong in the state. A high turnout of black voters, who made up 35 percent of voters in the 2004 Democratic primary, could clinch a Maryland primary win for Obama.
With two historic Democratic candidates and a tight race, election officials in both states and the District expect high voter turnout despite freezing temperatures and sleet in some areas.
"I voted for the next president of the United States, Barack Obama," said Phil Andonian, a 31-year-old lawyer sporting a Barack Obama T-shirt underneath his winter coat, outside a polling station in the nation's capital.
"The Clintons have shown themselves to be a political machine not much different than the Bush administration," Andonian said. "I think Obama is the one to bring about the kind of change we need in Washington."
Walking home from the elementary school where he voted, a 75-year old African-American man reflected on his vote for Obama.
"It's historic," he said, declining to give his name. "And this is probably the last time I'm gonna get to vote."
Post-Super Tuesday Momentum
Clinton visited her campaign headquarters in Virginia and did satellite television interviews, looking beyond Tuesday's trio of contests and touting the importance of a March 4 vote in Ohio.
"Ohio is really going to count in determining who our Democratic nominee is, and so I'm looking forward to getting there as soon as I can," she told ABC News' WCPO affiliate in Cincinnati.
In recent days Clinton's campaign has downplayed the importance of today's primary contests, focusing instead on the delegate-rich states of Ohio and Texas voting March 4 and Pennsylvania, which votes April 22.
Campaigning in the Capitol Hill neighborhood of Washington, D.C., Tuesday, Obama signed autographs, shook hands and thanked people who braved freezing temperatures to see him.
"Thank you everybody for voting, get all your friends to vote," Obama said.
Inside a Dunkin' Donuts in southeast Washington, the Illinois senator ordered a dozen mixed doughnuts and hot chocolates for campaign volunteers.
"Let me add to the tax revenue of D.C.," Obama quipped to District of Columbia Mayor Adrian Fenty, who has endorsed Obama.
The Fight for Delegates
At stake on Tuesday are 175 Democratic delegates, including seven from Democrats voting abroad, in a nominating race so tight it's become a state-by-state slog for delegates up to the party's national convention in August.
Clinton holds a razor-thin, 3-delegate lead, according to ABC News' delegate scorecard.
Obama stands at 1,173 delegates with Clinton at 1,176 delegates.
There are 113 GOP delegates up for grabs, and McCain has already won all of Virginia's 60 delegates because of the party's winner-take-all rules there.
Clinton Focuses on Ohio; Obama on Wisconsin
As the results from the East pour in, Clinton and Obama head West but in vastly different directions.
Clinton will rally in Texas as she focuses her campaign on the March 4 votes in Ohio, Texas, Vermont and Rhode Island -- states she hopes will be a firewall to Obama's post-Super Tuesday momentum. Obama, meanwhile, heads to Wisconsin, home of the next primary Tuesday, Feb. 19.
"Wisconsin is going to be a real battleground," Obama campaign manager David Plouffe told reporters on a conference call.
Plouffe said a win in Wisconsin, where Obama looks strong, would quiet opponents who have suggested Obama's support comes mainly from blacks, young voters, independents and high-income, mostly male, Democrats.
Over the course of the primaries and caucuses so far, Clinton has done well in large, diverse states, earning widespread support from low-income Democrats, women, senior citizens and Hispanics.
"By their own definition, Wisconsin would be a state with a lot of working-class voters, rural voters, a large state holding a primary that you would think would be prime turf for them," Plouffe said.
Hawaii, the state where Obama was raised, also votes next Tuesday.
McCain Battles Back Huckabee
With On the Republican side, McCain appears poised to sew up the Republican ticket, having 712 delegates of the 1,191 needed to win the GOP nomination, according to ABC News' delegate count.
Huckabee, with 234 delegates, badly trails McCain but has said he won't pull the trigger on his campaign until someone gets to the 1,191 delegate number.
"After tonight it is mathematically impossible for Governor Huckabee to secure the nomination," McCain communicators director Jill Hazelbaker told ABC News' Ron Claiborne.
McCain has built a near insurmountable lead in delegates, and a victory over Huckabee in Virginia has boosted his campaign.
A high turnout of Christian evangelicals and conservatives unhappy with the Arizona senator's record on immigration, taxes and his opposition to a federal ban on same-sex marriage made it a tight race there.
A third of voters in the Virginia Republican primary and three in 10 in Maryland describe themselves as "very" conservative, up sharply in both states compared with 2000 -- especially in Virginia, where "very" conservatives have nearly doubled as a share of the electorate, according to preliminary exit poll results, reports ABC News' Gary Langer.
Meanwhile, almost 50 percent of GOP voters are Christian evangelicals -- a group that has gravitated toward Huckabee, a former Baptist minister.
However McCain ultimately prevailed in a positive sign for his campaign going forward.
"With regard to Virginia, of course we'd like to win every race by wide margins, but we are accumulating the delegates we need to become the Republican nominee. Going forward, we will continue to work to unite the Republican party to defeat a liberal Democrat in November," Hazelbaker said.
Obama, Clinton Trade Swipes
Obama's post-Super Tuesday primary and caucus victories, combined with a month of stunning fundraising, have boosted the insurgent candidate's prospects.
Over the weekend, the Illinois senator won a slew of contests, defeating Clinton in a Louisiana primary as well as caucuses in Nebraska, Washington state and Maine.
Obama's campaign pulled in an astonishing $32 million in January and another $6 million arrived in the following 24 hours.
Clinton's campaign appeared unsteady this weekend when the campaign replaced campaign manager Patti Solis Doyle with Maggie Williams, another longtime adviser to the former first lady.
Clinton's campaign has maintained that she will do well in the delegate-rich states of Ohio and Texas that vote March 4, and the Pennsylvania primary April 22.
Hoping to distinguish themselves from the other before the Potomac primaries, Clinton and Obama took swipes at each other the day before the vote.
Clinton accused Obama of suspicious activity with a contributor.
"Sen. Obama has some questions to answer about his dealings with one of his largest contributors, Exelon, a big nuclear power company; apparently he cut some deals behind closed doors to protect them from full disclosure of the nuclear industry," she told ABC News' Washington affiliate WJLA last night.
Obama took a swipe at Clinton's campaign shakeup and news last week that the senator had injected $5 million of her own money into her campaign in January.
"I started from scratch and was up against an operation that had been built over the course of 20 years by a former president with the bulk of the Democratic establishment on their side and after setting up a hundred-million-plus operation with hundreds of employees around the country; it looks like we've played them to a draw so far," Obama said Monday night.
"I think that gives you a sense of how we run a campaign. There hasn't been a lot of drama in my campaign. You haven't seen a lot of turnover in my campaign."
Obama, Clinton Race May Go to Convention
Given the proportionality rules of the Democratic Party primaries, neither candidate may crush the other in the upcoming primary and caucus contests -- setting up a scenario in which the nomination fight spills onto the convention floor in August.
There, superdelegates -- 796 state party leaders, national party leaders and former Democratic presidents who get to act as free agents at the party's convention able to back any candidate they wish -- would hold the power.
While most of the superdelegates are sitting on the fence, Clinton is leading Obama among the superdelegates who have decided whom to support, according to ABC News' latest tally. Obama has emphasized his belief that it would be unfair if the Democratic contest was decided by superdelegates.
"We've got to make sure that whoever wins the most votes, the most delegates, that they are the nominee," he told ABC news affiliate WJLA.
"I think that would be problematic if either Sen. Clinton or myself came in with having won the most support from voters and that was somehow overturned by party insiders."
ABC News' Kate Snow, Ron Claiborne, Gary Langer and Karen Travers contributed reporting.