The Illinois senator, who is hoping to become the first black US president, beat Hillary Clinton, who is hoping to be the first woman, in all three contests.

But the close race continues - Mr Obama was always expected to do well in states which hold caucuses (Nebraska and Washington), states with well-educated, well-off voters (Washington) and those with a large black population (Louisiana), while Mrs Clinton hopes for successes later in Texas and Ohio on March 4.

Looking ahead to November's general election, Mr Obama said he would perform better than Mrs Clinton against the Republican party's presumptive nominee John McCain "because I have done better among independents in almost every single contest we've had".

He said both he and Mrs Clinton promised "a different kind of politics" for America and added: "We are going to be unified as Democrats, whoever the nominee, to make sure that we bring an end to the failed policies of George W Bush. That we can guarantee."

To applause, he said Mr McCain was a "good man and a genuine American hero" and added: "But understand that in this campaign, this year, he has made the decision to embrace the failed policies of George Bush's Washington."

He said the wheels had "fallen off" Mr McCain's Straight-Talk Express - the name of the Republican's campaign bus.

Speaking to Democrats at the Jefferson Jackson Day dinner in Richmond, Virginia - which goes to the polls on Tuesday - the Illinois senator said that when he started his campaign he knew he "wouldn't necessarily be Washington's favourite candidate".

"I knew we wouldn't get all the big donors, or all the endorsements right off the bat," he said. "I knew that I'd be the underdog in every contest from January to June. I knew it wouldn't be easy. But then some things started to happen."

He said Americans were "tired of being disappointed by our politics" and wanted change.