General Wesley Clark, the former Nato commander who entered the presidential race only four months ago, is emerging as the candidate most likely to upset Howard Dean's hitherto unstoppable drive towards the Democratic nomination in November.

Gen Clark's appeal was underlined by the $11m (£6m) he raised in the fourth quarter of 2003, putting him second only to the former Vermont governor in the vital "invisible primary" of fundraising. Mr Dean collected $14m in the same period.

But armed with $3.7m of additional matching funds from the federal government, Gen Clark is now able to finance a vigorous campaign well beyond the crucial early contests in Iowa and New Hampshire. Today the Democratic candidates gather in Iowa's capital, Des Moines, for their latest campaign debate, just 15 days before the state's caucuses which raise the curtain on a tightly packed primary season.

A new poll this weekend gives Mr Dean a commanding 10-point lead nationally over his rivals, but in Iowa - where Gen Clark is not competing - he faces a much closer race.

Though the caucuses are notoriously unpredictable, the signs are that the former Vermont governor is only marginally ahead, at best, of Richard Gephardt, the former Democratic House leader whose power base is in neighbouring Missouri.

With strong links to organised labour and his vast Washington experience, Mr Gephardt has long been regarded by an anxious Democratic establishment as a possible "stop Dean" candidate. But if he loses in Iowa, his camp admits that his candidacy will be over. Campaigning in New Hampshire last week, Gen Clark tried to cement the impression that he was the one viable alternative to Mr Dean, whose shoot-from-the-hip style and lack of foreign policy experience have many Democratic elders alarmed that he will be trounced by Mr Bush in the election. This would destroy Democratic prospects of recapturing control of Congress in the process.

"I'm one of only two candidates in a position to win the nomination," he said. "I am best able to stand up to George W Bush and win the debate about who can make our country secure over the next four years."

Party strategists believe that after 3 February, when South Carolina and five other states hold primaries, opposition to Mr Dean will crystalise around one candidate.

Gen Clark, with his background in national security and links to the party's centrist establishment, led by ex-president Bill Clinton, fits the bill perfectly. In a back-handed compliment to Gen Clark's progress, other candidates are starting to turn their fire on him as well as Mr Dean. They point to his lack of experience in politics and accuse him of leaning too heavily on his military record.

The Clark campaign is sending a 15-minute video about the general to 50,000 possible primary voters in New Hampshire. It will also be shown on cable channels in the state before the 27 January primary.