Senator Hillary Clinton has defeated Barack Obama in the Democratic party's presidential primaries in Texas and Ohio.
Her performance in the two crucial contests revitalises her campaign after 12 defeats in a row and is likely to prolong the race for the party's nomination to contest the presidential election in November.
She also won Rhode Island, while Senator Obama won the state of Vermont.
Speaking before a crowd of supporters in the Ohio state capital Columbus, Senator Clinton said that 'as Ohio goes, so goes the nation'.
'The people of Ohio have said it loudly and clearly, we're going on. We're going strong. And we're going all the way!' she said to huge cheers from her supporters.
The race in Texas was much closer. Mr Obama addressed supporters at an outdoor rally in San Antonio before the result was called. He congratulated Ms Clinton for her win in Ohio but said his campaign would still hold the same lead in delegates.
The Democratic contest will now centre on Wyoming and Mississippi, with the next major primary in Pennsylvania in April.
McCain promises determined campaign
Senator John McCain finally won the Republican party nomination. He is expected to be endorsed by US President George W Bush in Washington later today.
Mr McCain said he will now make a 'respectful, determined and convincing case' of why he should be elected in November.
In a victory speech after winning Texas, Ohio, Vermont and Rhode Island, he said the next US president must explain how to bring the Iraq war to its 'swiftest possible conclusion' without exacerbating a sectarian conflict that could destabilise the Middle East.
Mr McCain also said he would help repair the ailing US economy and strengthen job growth by pushing lower taxes, less government regulation and resist trade protectionism.
'I will leave it to my opponent to claim that they can keep companies and jobs from going overseas by making it harder for them to do business here at home. We will campaign to strengthen job growth in America by helping businesses become more competitive with lower taxes and less regulation,' he said.
Former Arkansas governor Mike Huckabee quit the White House race and congratulated Mr McCain on winning the Republican Party's nomination.
Mr Huckabee, who had fought a surprisingly strong campaign for a rank outsider, threw his weight behind Mr McCain saying it was time for the party to unite behind a candidate.
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