Barack Obama inflicted shattering defeats on Hillary Clinton tonight in Washington state and Nebraska, beating her by a margin of two to one.
He was also won Louisiana, though by a narrower margin.
Although votes were still being counted, the projected gap between Obama and Clinton was large enough for US television companies to call all three contests in his favour.
To complete his night, he also picked up a win in a caucus in the US Virgin Islands.
In the Republican race, there was embarrassment for John McCain only days after declaring himself to be the Republican nominee. He suffered an overwhelming defeat in Kansas at the hands of Mike Huckabee, the former governor of Arkansas, and was involved in a race too close to call in Louisiana.
Obama's victories tip the balance in his favour in the battle to rack up delegates, who will choose the Democratic nominee at the party convention in August.
A total of 161 delegates - about 8% of the 2,025 delegates needed to win the nomination - was at stake in the Louisiana primary and caucuses in Nebraska, Washington state and the US Virgin Islands.
Obama took the bulk of the delegates.
Clinton began Saturday with a slender delegate lead. In The Associated Press count, she had 1,055 delegates to 998 for Obama.
The most important of the contests was populous Washington state, which the Clinton team had expected would produce a closer result.
Turnout in Washington and Nebraska was high, reflecting the excitement generated by the Democratic race. Party officials said turnout was about double that in 2004.
The wins in Washington and Nebraska demonstrated again Obama's ability to reach out to whites. Both Nebraska and Washington are 90% white.
With 99% of the votes counted in Nebraska, Obama had 66% to Clinton's 32%. In Washington, with 96% of the vote counted, he had 66% to her 31%.
In Louisiana, with 63% of the vote counted, he had 53% to her 39%.
In the Republican race in Louisiana, with 63% of the votes counted, Huckabee had 47% to McCain's 39% and in Washington, with 37% counted, Huckabee had 27% to McCain's 23%, with Mitt Romney also polling strongly even though no longer in the race, with 18%, and Ron Paul on 21%.
The closeness of the race compared with the other two could reflect the displacement of many African-Americans from Louisiana after Hurricane Katrina more than two years ago.
About five out of six of African Americans, who made up 45% of the voters, supported him.
The Clinton team, before the polls closed, tried to play down the contests, attributing their expected defeats in part to his outspending them on advertising: $300,000 more in Louisiana on television ads, $190,000 more in Nebraska and $175,000 more in Washington state.
The Clinton team is anticipating further defeats on Tuesday in Maryland, Virginia and Washington, DC.
The Clinton team said: "Although the next several states that hold nominating contests this month are more favourable to the Obama campaign, we will continue to compete in them and hope to secure as many delegates as we can before the race turns to Ohio, Texas(both March 4) and Pennsylvania (April 22)."
In the Republican race, Huckabee laid bare Senator McCain's problems with the conservative wing of the Republican party by winning overwhelmingly the Kansas caucuses, with 60% to McCain's 24%.
The Democrats held their caucus in the state on Tuesday.
Huckabee said he would stay in the race in spite of pressure from Republican governors and congressmen to stand aside for the sake of party unity.
Huckabee said the Republican nomination should be an election, not a coronation. "While people in Washington gravitate towards the senator's campaign, people across the US are gravitating towards us," he said.
The McCain team responded that it had not expected to win Kansas and that McCain remained the presumptive nominee.
Ron Paul, the only other Republican left in the race, took 11%. His team announced tonight they are to scale back campaign staff and concentrate his efforts in Texas where he is seeking re-election to the Congress.
Huckabee captured all 36 delegates that were at stake.
Including the Kansas results, McCain has 719 delegates, Huckabee 234, and Paul 14.
Earlier in the day, at a conservative conference in Washington, DC, Huckabee, a Baptist preacher, challenged the view that he was too far behind McCain in the delegate count. "I didn't major in math. I majored in miracles, and I still believe in them," Huckabee said.