The Democratic chairman called on his party yesterday to stop the contest between Barack Obama and Hillary Clinton by July 1, giving warning that “there'll be some nasty fights if it goes to the convention — and people will walk out”.

Howard Dean's attempt to set a deadline — eight weeks before the convention in Denver — is symptomatic of the alarm felt by Democratic leaders over the damaging drawn-out battle between the candidates continuing over the summer.

After a week of increasingly vitriolic exchanges between the rival campaigns, Mr Dean told both sides to “keep their mouths shut” and realise how harmful they were being to the party's prospects of winning the White House in November.

Recent polls suggest that more than a quarter of Mrs Clinton's supporters would defect to the Republicans if Mr Obama wins. Almost a fifth of his voters threaten to switch if she becomes the Democrat nominee. “We do need to keep in mind that personal attacks now often do have the seeds of demoralisation later on,” Mr Dean said. “You do not want to demoralise the base of the Democratic Party by having the Democrats attack each other.”

Although Mr Obama has an almost insurmountable lead among elected delegates from primaries and caucuses, he cannot win the nomination without a sizeable chunk of the 795 super-delegates. Mr Dean said that about 450 of the super-delegates had already pledged their vote to one of the candidates. “I'd like the other 350 to say who they're for at some point between now and the first of July so we don't have to take this into the convention,” he added.

Mrs Clinton hopes that, by raising doubts about her opponent's electability, she can persuade super-delegates to overturn the will of the electorate. Mr Obama's campaign described this as a “scorched-earth tactic”.

In a sign of her determination to carry on beyond the last big primary in Pennsylvania next month, either Mrs Clinton or her husband, Bill, have made campaign stops this week in later states, including West Virginia, Kentucky, North Carolina and Indiana. In an interview published yesterday, she scorned calls for her to drop out, saying: “The people who are supporting me sure don't want to see it over. They want me to keep fighting.”

Mrs Clinton still harbours faint hopes that delegates from Florida and Michigan, the majority of whom back her, will be seated at the convention despite being banned because the states broke party rules by holding primaries in January.

Asked how she could get the punishment lifted, she said: “We can always go to the convention - that's what credentials fights are for.”

Mr Dean is said to be talking about how to broker a pre-convention peace deal with a group of uncommitted party elders, including John Edwards — who dropped out of the race in January — as well as Jimmy Carter, the former President, Harry Reid, the Senate Leader, and Al Gore.

There has even been speculation that if Mr Obama's credibility is badly damaged by August, Mr Gore, a Nobel Peace Prize winner, could yet emerge as a compromise candidate. Mr Obama was boosted yesterday by the surprise backing of the Pennsylvania senator, Bob Casey, himself a super-delegate and an important endorsement in a state where he trails Mrs Clinton, especially among white, working class or older voters.

Another senator supporting Mr Obama called on Mrs Clinton yesterday to quit the race. Patrick Leahy said: “I am very concerned. I think that her criticism is hurting him more than anything John McCain has said.”

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