Veteran senator Edward Kennedy has delivered an impassioned endorsement of Barrack Obama at the opening day of the United States Democratic Party convention in Denver on Tuesday.
Mr Obama is set to become the first African-American nominee for president.
Mr Kennedy, who is suffering from brain cancer, took to the stage in a surprise appearance amid emotional scenes to declare that Mr Obama would bring change and hope.
Mr Obama's wife, Michelle, a keynote speaker at the convention, later began her speech, saying her husband would make an "extraordinary president" who would give all children the chance to fulfill their dreams.
She sought to reassure the cheering crowd that he shared the values of all ordinary Americans.
She urged the party to unite in its bid to secure the presidency. The speech is being broadcast nationwide on prime-time television.
His defeated rival for the candidacy, Hillary Clinton, says the Democrats are united, although some Clinton supporters at the conventions say they will not vote for Mr Obama.
The party is looking to use the four-day event to present the Illinois senator's personal side and heal the rifts of the bruising primaries race.
More than 4000 Democratic delegates and tens of thousands of officials, activists, protesters and journalists are in Denver. The schedule is packed with speeches from party seniors and Obama relatives.
Mr Obama, 47, will formally accept the party's nomination when he addresses a crowd of 80,000 at a sports stadium on Thursday night, fresh from touring electoral battlegrounds.
The convention has two goals, said Bill Burton, a spokesperson for Mr Obama.
"We want to make sure people know exactly who Senator Obama is and where he wants to take the country, and two, that voters know their choice in this election."
Party officials downplayed any scope for discord after Mr Obama chose veteran Senator Joseph Biden as his running mate.
Some of Mrs Clinton's supporters are said to be angry that she was never seriously considered for the role. But the official party line is that all is sweetness and light in Denver, the BBC reports.
Mr Burton says the presidential hopeful had been in touch with both Mrs Clinton and her husband, former president Bill Clinton, and that everyone was on board for party unity.
However, the BBC reports there is a potential flash-point on Wednesday night. A roll-call of delegates will be taken and Mrs Clinton's supporters will get the chance to make a symbolic effort to have her nominated. It appears that she has asked them to switch their formal allegiance to Mr Obama but some Clinton supporters may not play ball.
They will be aware of the revelation over the weekend that Mrs Clinton was never legally vetted as a vice-presidential candidate, meaning she was never seriously considered for the post.
A Wall Street Journal/NBC poll published last week suggested that only 52% of people who voted for Mrs Clinton during the primaries had so far decided to vote for Mr Obama in November. About a fifth said they would vote for the Republican nominee John McCain, while 27% had not yet decided how they would vote.
Mrs Clinton says the stakes are high for the election, to be held on 4 November, and the US could not afford a win by Mr McCain because he would follow in the footsteps of the unpopular President George Bush.