Barack Obama has won three more states in the battle for the Democratic Party's presidential nomination.
The Illinois senator won in Louisiana, Nebraska and Washington state and won caucuses in the US Virgin Islands.
Mr Obama is almost neck-and-neck with Hillary Clinton in the nationwide battle to be the party's nominee.
For the Republicans, Mike Huckabee beat frontrunner John McCain in Kansas and Louisiana. Mr McCain was declared the winner in Washington, reports said.
'Suspicious of McCain'
The BBC's James Coomarasamy, in Washington DC, says Mr Obama's easy victories in the Democratic contests will not be decisive, but they will return him the initiative.
In a speech to Democrats in Virginia, Mr Obama said the momentum of the day's results would enable him to win his next contest, in Virginia on Tuesday.
"Today the voters from the west coast to the Gulf coast, to the heart of America, stood up to say 'yes we can'... we won north, we won south, we won in between."
As for the Republicans, our correspondent says that Mr Huckabee's victories in Kansas and Louisiana show that there is a socially conservative section of the Republican Party that is very suspicious of John McCain, so Mr McCain has some work to do to unite his party.
Mr Huckabee and third-placed Ron Paul have been coming under pressure to step aside for the sake of party unity.
Mr Huckabee said on Saturday that he had no intention of pulling out.
"Am I quitting? Let's get that settled right now. No, I'm not," he said.
"I majored in miracles, and I still believe in them."
Going into Saturday's contests, Mr McCain had a wide lead with 719 delegates, to Mr Huckabee's 198 and Mr Paul's 14.
Mr Romney's suspended campaign still has 298 delegates.
On the Democratic side, Mr Obama and Mrs Clinton are facing the prospect of a long drawn-out battle after neither was able to deliver a knock-out blow in Super Tuesday's 22 state contests.
Fight for funds
In advance of the 9 February contests Mrs Clinton had won 1,055 delegates to Mr Obama's 998 of the 2,025 needed to secure victory at the Democratic party convention in August.
RESULTS SO FAR
Hillary Clinton: 1095 delegates, 13 states
Barack Obama: 1070 delegates, 18 states
John McCain: 719 delegates, 12 states
Mike Huckabee: 234 delegates, 7 states
Ron Paul: 14 delegates, 0 states
Mr Obama's success in Washington, Louisiana and Nebraska will add to his delegate tally and buoy up his supporters.
But Mrs Clinton's campaign says that she expects to take victory at the forthcoming primaries in Maine on 10 February and in Texas and Ohio on 4 March.
As well as fighting for every vote, the candidates are also jostling for funds. The Obama campaign says it has raised $7m (£3.6m) since Tuesday.
The Clinton camp is now claiming a similar fund raising bump, gaining $6.4m. They earlier admitted that the former first lady had lent her campaign $5m to paper over what aides called a "temporary cash flow problem".