Sen. was projected to sweep Sen. Hillary Rodham Clinton in Tuesday's primaries in Maryland, Virginia and the District of Columbia, NBC News said, giving him an unbroken string of victories since Super Tuesday.
On the Republican side, Sen. John McCain defeated former Arkansas Gov. Mike Huckabee in all three contests.
The three wins — which came by substantial margins — helped Obama build on the delegate lead that he has gained in the past week in NBC News' count.
“Tonight we’re on our way,” Obama told cheering supporters Tuesday night in Madison, Wis. “We now have won east and west and north and south and across the heartland of this country we love.”
Clinton was said to be depending on victories in the big states of Ohio and Texas next month in her struggle to keep up in a race she once commanded.
She campaigned in El Paso, Texas, on Tuesday night, telling supporters: "I'm tested, I'm ready — let's make it happen."
Clinton didn't mention Tuesday's results, but there were lingering signs of the disquiet in her campaign.
Clinton's campaign manager resigned over the weekend, and it was learned Tuesday that deputy campaign manager Mike Henry resigned Monday. A source told NBC that Henry was leaving to allow new campaign manager Maggie Williams to organize her own team.
Obama makes some inroads With his victories Tuesday, Obama showed signs of eating into Clinton's voter base. He had nearly two-thirds of the vote in Virginia, about 60 percent in early returns in Maryland and 75 percent in the District of Columbia.
Interviews with voters leaving the polls showed Obama split the white vote with Clinton in Virginia, though she won it by 10 percentage points in Maryland. She won a majority of white women in both states, though by less than she is accustomed to. He won among white men in Virginia, and they split that vote in Maryland.
In addition to his usual strong showing among young voters, Obama was also running about even among those over 65, a group Clinton usually dominates.
"This is the new American majority. This is what change looks like when it happens from the bottom up," Obama said at the campaign rally in Wisconsin, which holds its primary next Tuesday.
Obama, who would be the first black president, also won the votes of nine in 10 black voters in Virginia, where they were about a third of the electorate, and his won almost as many in Maryland.
And Obama was winning 66 percent to 33 percent among independents, who made up a fifth of the Democratic electorate in Virginia. He did even better — 70 percent to 26 percent — among Republicans, who made up 8 percent of the Democratic vote. Virginia held the two parties' primaries on the same day for the first time and voters can cross party lines in primaries there.
Virginia Gov. Tim Kaine said in an interview on MSNBC that the makeup of the Democratic electorate bore out his belief that Obama could strongly attract independent voters in November in Virginia, which has been solidly Republican in previous presidential elections.
“The results tonight suggest that we’re going to be very razor-thin competitive in November,” said Kaine, who has endorsed Obama.
Clinton looks to Texas
The Illinois senator won a string of contests in all regions of the country over the weekend, routing Clinton in a Louisiana primary as well as caucuses in Nebraska, Washington state, Maine and the Virgin Islands.
Obama's latest victories put more pressure on Clinton, who faces possible defeats next week in Wisconsin and Hawaii. Obama led Clinton on Tuesday night, with 1,078 delegates to her 969, according to NBC News.
Clinton hopes to respond with victories in Texas and Ohio on March 4, states where both candidates have already begun television advertising.
“Clearly, coming out of Super Tuesday, it was expected that Sen. Obama would take the states he has taken,” said Lisa Caputo, a senior adviser to Clinton.
Caputo acknowledged in an interview on MSNBC that the campaign was “focusing its resources on the big states with the big delegate counts, because let’s remember, it’s all about the delegates.”
“All eyes are on the states of Texas and Ohio,” she said. “She absolutely has to win Ohio and Texas, and probably Pennsylvania.”
While still in Virginia on Tuesday, Clinton did satellite interviews with 10 TV stations in Ohio, Texas and Wisconsin, calling for more debates and addressing regional concerns such as the economy in Ohio and immigration in Texas.
Asked about the possibility of sharing the November ticket with Obama — before Tuesday's results were known — she said it was too soon to talk about such things, but in an interview with WTMJ in Milwaukee she echoed the comment her rival has been making about her: "I have the highest regard for him. He was my friend before this started, and he will be my friend going into the future."
In Virginia, the parties held binding primaries on the same day for the first time.
“We have had heavy voter turnout ... throughout the state,” Susan Pollard, a spokeswoman for the Virginia Board of Elections, told NBC affiliate WRC of Washington.
In the Washington suburb of Alexandria, the previous primary record was broken by noon, WRC reported. Most were voting in the Democratic primary.
More than a third of voters in the Virginia Democratic primary said they had not voted in a primary before, as did almost one in five voters in the Maryland Democratic primary, according to the exit interviews.
In Maryland, icy weather caused traffic problems and a judge agreed to order that voting places stay open an extra 90 minutes. Ballots cast during the extended 90 minutes will be provisional ballots, as required by federal law. That means people will vote on paper, the ballots will be put in an envelope and reviewed after the election. An election official says the provisional ballots won't be counted until Tuesday, because Monday is a holiday.
But they shouldn't affect the outcome, considering the size of Obama's lead.
Kweisi Mfume, a onetime Democratic representative from Maryland and former president of the NAACP, noted exit polls in Virginia that suggested strong support for Obama among white and Latino voters. “We really have, to some extent, come of age,” he said in an interview on MSNBC.
“I think Barack Obama has a momentum that is going on not just in Maryland, not just in Virginia, but nationwide,” said Mfume, who has not endorsed a candidate.
Tom Schaller, a political science professor at the University of Maryland-Baltimore County, told NBC affiliate WRC of Washington that Obama’s message of change was effective with people who perceived Clinton as an extension of her husband’s administration.
“She’s running against the Bush era, and he’s running against the Bush and Clinton era,” Schaller said.