Illinois Senator Barack Obama looks set to overtake his rival, Senator Hillary Clinton, in the race for the Democratic presidential nomination.
In the latest round of primaries, Mr Obama has clinched Virginia, Maryland and the District of Columbia.
But neither has secured enough party delegates to secure the nomination to contest November's presidential poll.
For the Republicans, Senator John McCain is on course to win all three races, beating rival Mike Huckabee.
With eight consecutive primary wins behind him, Mr Obama is beginning to look formidable and the manner of his victory on Tuesday looks ominous for Mrs Clinton, says the BBC's North America editor, Justin Webb.
"Tonight we're on our way," Mr Obama told supporters in Wisconsin, which will hold the next primary on 19 February.
Hillary Clinton 17 states, 1,592 delegates
Arizona, Arkansas, California, Florida, Massachusetts, Michigan, Nevada, New Hampshire, New Jersey, New Mexico, New York, Ohio, Oklahoma, Pennsylvania, Rhode Island, Tennessee, Texas
Barack Obama 24 states, 1,723 delegates
Alabama, Alaska, Colorado, Connecticut, Delaware, Georgia, Hawaii, Idaho, Illinois, Iowa, Kansas, Louisiana, Maine, Maryland, Minnesota, Missouri, Nebraska, North Dakota, South Carolina, Utah, Vermont, Virginia, Washington state, Wisconsin
2,025 delegates needed for nomination. Source AP (includes all kinds of delegates) Q&A: US election delegates
Arizona, California, Connecticut, Delaware, Florida, Illinois, Maryland, Missouri, New Hampshire, New Jersey, New York, Ohio, Oklahoma, Pennsylvania, Rhode Island, South Carolina, Texas, Vermont, Virginia, Washington state, Wisconsin
Mitt Romney 11 states, 251 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)
Mrs Clinton, who has recently lost a number of key staff, indicated she would be focusing on the races in Ohio and Texas in March as her best hope to retake the lead.
"We're going to sweep across Texas in the next three weeks," she said, making no mention of the three contests she lost.
"I'm tested, I'm ready, let's make it happen," she told supporters.
Each Democratic candidate is about halfway to winning the 2,025 delegates needed to secure victory at the party's national convention in August.
With most of the Tuesday's votes counted, Mr Obama has edged into the lead with 1,223 delegates to Mrs Clinton's 1,198 delegates, according to the Associated Press news agency.
Mrs Clinton's deputy campaign manager, Mike Henry, reportedly stepped down on Monday, in a shake-up coming only a day after her campaign manager left.
Meanwhile Mr McCain, who holds a strong lead over his Republican rival Mike Huckabee, told supporters he was "fired up and ready to go".
With results counted in almost all of Virginia's precincts, Mr McCain led by 50% to Mr Huckabee's 41%.
In the District of Columbia, Mr McCain took 68% of the Republican vote to 17% for Mr Huckabee, with almost all the votes counted. Congressman Ron Paul took 8%.
We are approaching the end of the first half of this election on quite an upswing