John Kerry has won easy victories in the latest state caucuses to find a Democrat challenger to President Bush.
Kerry takes the primaries but the actual election may be very different
He secured about half the vote in Washington State and Michigan, beating Howard Dean into second place.
He is also tipped to win today's Maine caucuses but may face a tougher race in southern states on Tuesday.
With nine wins from the first 11 primaries and caucuses, Mr Kerry's victory speech largely ignored his five rivals for the nomination.
"George Bush's days are numbered, and change is on its way," he told a Democratic dinner in Virginia.
Wesley Clark and John Edwards - the two Kerry rivals who have to date won one primary apiece - have largely held their campaigning fire for the polls due in Virginia and Tennessee on Tuesday.
The BBC's Rob Watson says they are trying to make the case to the Democratic Party faithful that their nominee cannot win a presidential election without doing well in the South.
One-time front-runner Dean has said he will withdraw from the race if he does not win the Wisconsin ballot on 17 February.
The former Vermont governor took a new blow on Saturday when the head of a major union - the American Federation of State, County and Municipal Employees - withdrew his support.
Kerry's commanding lead
Mr Kerry's victories have lengthened his lead over his rivals in the number of delegates pledged to vote for him at the party's convention.
His showing swelled his total to 411, with Mr Dean at 175, Mr Edwards at 116 and Mr Clark at 82.
Michigan had 128 delegates at stake in caucuses, and Washington offered another 76. In Maine, there are 24 delegates at stake.
The BBC's David Bamford says it is hard to see how any of Mr Kerry's rivals can derail him now but a potential political brick wall still lies ahead of him in the shape of George W Bush.
It is virtually impossible, he notes, to unseat a serving president seeking a second-term re-election unless you have a perfect candidate running a perfect campaign or the president has been having a disastrous time in office.
Mr Bush himself has given a rare interview to American television, due for broadcast on Sunday, in which he answers questions about Iraq among other things.
The caucuses in Michigan saw many Democrats voting over the internet for the first time.
About 46,000 such votes were registered, party officials told AFP news agency.
Mr Kerry's opponents want to keep the race alive, at least until next month's "Super Tuesday" when more than half of the 2,162 delegates needed to ensure victory can be won.
Mr Edwards, a North Carolina senator, vowed to stay in the race until he won his party's nomination.
Vietnam war veteran Mr Kerry said on Saturday he was looking forward to the fight in the southern states.
"I'm not worried about coming down South and talking to people about jobs and schools and health care and the environment," he told supporters in Nashville, Tennessee.
Most delegates who vote for a candidate at the national presidential nominating convention are allocated according to a candidate's support in state-wide caucuses or polls; some delegates are assigned for party leaders and elected officials to allocate.