On a primary winning streak, Sen. Barack Obama, D-Ill., swept the so-called Potomac primaries last night, overwhelmingly defeating Sen. Hillary Clinton, D-N.Y., in Democratic contests in Maryland, Virginia and Washington, D.C.
For the first time, the Illinois senator has taken the lead over Clinton in the ABC News overall delegate estimate.
This is the eighth straight victory for Obama, who is increasingly taking on the mantle of Democratic frontrunner.
"Today, the change we seek swept through the Chesapeake and over the Potomac. We won the state of Maryland. We won the Commonwealth of Virginia. And though we won in Washington D.C., this movement won't stop until there's change in Washington," Obama told supporters at a rally in Madison, Wisc. last night.
"We are bringing together Democrats and Independents and Republicans; blacks and whites; Latinos and Asians; small states and big states; Red States and Blue States into a United States of America," Obama said. "This is the new American majority."
On the Republican side, Sen. John McCain, R-Ariz, won primaries in Maryland and Washington, D.C. and battled back insurgent candidate Mike Huckabee, the former Arkansas governor, in a Virginia primary made close by a high turnout of conservatives and Christian evangelicals.
"He certainly keeps things interesting, a little too interesting at times tonight," McCain said of Huckabee at a campaign rally in Alexandria, Va. "Makes it more interesting."
Clinton's defeat in the Virginia primary crushed what may have been her best chance at a Potomac primary win.
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, winning demographic groups once thought to be Clinton's core.
Ignoring her losses and her Democratic rival, Clinton addressed a campaign rally in El Paso, Texas -- one of the states she is focusing her campaign on winning March 4.
"I need you to stand up for me because, because if we stand up together, if we work together, if we fight together, we will take back America and we will make history together," Clinton said.
In a sign of further turmoil within Clinton's campaign, word came late tonight that Clinton's 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.
Obama swept to victories in Virginia, Maryland and D.C.where majorities of Democratic voters said the top attribute they are seeking in a candidate is the one who can "bring needed change," according to preliminary exit poll results -- a message consistently promoted by Obama.
Obama won 88 and 89 percent of African-American voters in Virginia and Maryland and rode majority support from white men.
White voters in Virginia favoring him over Clinton by a 14-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.
McCain, who is trying to court conservatives unhappy with his record on taxes, immigration and his opposition to a federal ban on same-sex marriage, battled back Huckabee in Virginia, where a high turnout of Christian evangelicals and conservatives made it a tight race.
Thirty-four percent of Republican primary voters in Virginia described themselves as very conservative, putting Huckabee in the game, reports ABC News' Gary Langer.
He won very conservative voters over McCain by better than a 3-1 margin, while McCain narrowly prevailed among somewhat conservatives and won moderate Republicans by more than 2-1, according to exit polls.
McCain, who had an insurmountable delegate lead going into today's primaries, has picked up hundreds more delegates thanks to Virginia and Washington, D.C.'s winner-take-all rules.
Despite his continued losses, Huckabee vowed again tonight not to pull the trigger on his campaign until someone reaches the 1,191 GOP delegates needed to win the Republican nomination.
"The nomination is not secured until someone has 1,191 delegates," Huckabee said at a news conference in Little Rock, Arkansas tonight.
"It would be premature to quit to quit the game," he said. "I've not been one who believes you believe the field because it has gotten difficult. You keep playing last second of the clock."
McCain's campaign tonight said Huckabee was on a losing quest.
"After tonight it is mathematically impossible for Governor Huckabee to secure the nomination," McCain communicators director Jill Hazelbaker told ABC News' Ron Claiborne.
In his victory speech tonight, McCain appeared to deliver a message to those conservatives who doubt him.
"As I have done my entire career, I will make my case to every American who will listen," McCain said. "I will not confine myself to the comfort of speaking only to those who agree with me ... I will fight every moment of every day for what I believe is right for this country, and I will not yield."
Despite freezing temperatures, rain and sleet in some areas, Democrats energized by historic candidates and the tighest nominating race in recent history turned out in high numbers.
"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."
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.
Today Clinton kept a low profile, visiting her campaign headquarters in Virginia and doing 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.
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 the city," Obama quipped to District of Columbia Mayor Adrian Fenty, who has endorsed Obama.
At stake on Tuesday were 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.
Going into the Potomac primaries, Clinton held a razor-thin delegate lead, according to ABC News' delegate scorecard.
But Obama surpassed Clinton tonight as the results continued to pour in.
Meanwhile McCain picked up a big chunk of the 113 GOP delegates up for grabs, thanks to Virginia and Washington, D.C.'s winner-take-all rules.
After the contests in the East, Clinton and Obama head West but in vastly different directions.
Clinton left D.C. for 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, will deliever a speech on the economy tomorrow in 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 Tuesday.
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.
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 Monday 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.
"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."
Despite Obama's surge, neither candidate may crush the other in the upcoming primary and caucus contests given the proportionality rules of the Democratic Party primaries -- 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 survey.
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 Monday.
"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, Karen Travers, Tahman Bradley, and Sunlen Miller contributed reporting.