Mr John Kerry captured the Democratic nomination to challenge President Bush in November, scoring a string of coast-to-coast wins last night that knocked rival Mr John Edwards out of the race.
Mr Kerry, who received a call of congratulations from Mr Bush, quickly turned his focus to the general election and said he was "a fighter" who would challenge the president on a range of issues.
"Tonight the message could not be clearer, all across our country, change is coming to America," Mr Kerry told supporters in Washington. "With one united Democratic Party, we can and we will win this election."
The Massachusetts senator continued his domination of the Democratic race on its biggest night of voting, scoring wins in nine states including victories in Georgia, Minnesota and Ohio, which had been targeted by Mr Edwards.
The wins capped a spectacular political resurrection for Mr Kerry, whose campaign was considered dead just two months ago but charged back to life as Democrats began to evaluate which candidate stood the best chance to beat Mr Bush in November.
Mr Kerry and Mr Bush now embark on what promises to be a hard-fought, eight-month general election campaign. Mr Bush, who trails Mr Kerry in some opinion polls, launches his first television ads in 17 battleground states tomorrow as he starts to spend a more than $100 million campaign war chest.
Mr Edwards had hoped to slow Mr Kerry's march to the nomination, but narrowly lost Georgia and was swamped by Kerry in most of the other states voting on "Super Tuesday."