Hillary Clinton still trails Barack Obama in terms of delegates won

Mrs Clinton's campaign was revived when she won key primary votes on Tuesday, but Mr Obama still holds a slight lead.

With the race so tight, pressure is building for Michigan and Florida - who were stripped of votes after they broke party rules - to hold fresh primaries.

Party head Howard Dean has urged state party officials to present a new plan.

Speaking on US network CNN, he also warned that the tone of the campaign "may get nastier" and said discussions would take place to try to prevent that happening.

With the numbers so close, the debate over what should happen in Michigan and Florida may become heated.

Both states were told their delegates would not be seated at the party's August national convention - meaning they cannot vote on who should be the Democratic presidential candidate - after they breached party rules by holding primaries before 5 February.

According to the Associated Press news agency, Mr Obama had 1,567 delegates and Mrs Clinton 1,462, as of Wednesday. It takes 2,025 to secure the party's nomination.

'Voices silenced'

State Democratic Party officials who had previously insisted the original results in Florida and Michigan should stand have now indicated they may be willing to hold new contests.

DEMOCRATIC DELEGATE RACE BARACK OBAMA: 1,567 Delegates won on 4 March: 173 States won: 24 HILLARY CLINTON: 1,462 Delegates won on 4 March: 185 States won: 16 Delegates needed to secure nomination: 2,025. Source: AP at 0530 GMT Delegates: Who's ahead Q: What next? Dirty dogfight

Florida's Republican Governor Charlie Crist and Michigan Governor Jennifer Granholm, a Democrat backing Mrs Clinton, added to the pressure with a joint statement on Wednesday.

"We must restore the rights of the more than 5m voters whose voices have been silenced," they said.

Aides to Mrs Clinton - who won the popular vote in both states, although Mr Obama's name was not on the ballot in Michigan - would also be open to the idea of repeated ballots.

Speaking to reporters on Thursday, Clinton communications director Howard Wolfson said: "Given the results of the primaries that were held there, we would feel good about our prospects in those states."

He warned that not representing primary voters there could disadvantage the party in November's general election.

One stumbling block is how new primaries would be paid for, with Mr Dean stressing that the national party needed to keep its funds for fighting the general election campaign.

Mr Obama's team has previously opposed suggestions from Mrs Clinton's camp that the delegates from the two states should be awarded according to the ballots already held.

Breathing space

Meanwhile, Republican candidate John McCain was making appearances in Florida and Georgia on Thursday.

President George W Bush said he would back Mr McCain's campaign

Speaking in West Palm Beach, Florida, with state governor Mr Crist by his side, he sidestepped questions about who he planned to pick as his vice-presidential running mate.

"You know, obviously, we have just begun that process, and we, in fact, have not even outlined how we're going to go about this," he told reporters.

Mr McCain was endorsed by US President George W Bush at the White House on Wednesday, a day after clinching the Republican Party's nomination in Tuesday's primaries.

He has breathing space to map out his strategy to beat the eventual nominee in November, correspondents say.

However, he must also ensure that with the media focused on the battle between Mr Obama and Mrs Clinton, his own general election campaign does not fade from view.

Next contests

His Democratic rivals are turning their attention to the forthcoming contests and to winning over undecided super-delegates, party officials who will have a vote at the national convention.

NEXT DEMOCRATIC VOTES 8 March: Wyoming caucus, 12 pledged delegates 11 March: Mississippi primary, 33 pledged delegates 22 April: Pennsylvania primary, 158 pledged delegates 6 May: Indiana primary, 72 delegates; North Carolina primary, 115 pledged delegates In quotes: Reaction to votes US media reaction Who won it for Hillary

Mr Obama, who won 11 contests in succession in the month before the 4 March polls, has pointed out that Mrs Clinton "barely dented" his lead in the delegate count, despite her three wins on Tuesday.

He himself won Vermont and, because the Democratic Party allocates delegates proportionally, he and Mrs Clinton will share out the delegates from all four states.

Democrats in Wyoming will take part in a caucus on Saturday, with 12 pledged delegates at stake, and Mississippi has its primary next Tuesday, with 33 Democratic delegates to be awarded.