A leading Democratic super-delegate switched his support from Hillary Clinton to Barack Obama yesterday, saying that their long nomination battle had made one White House hopeful particularly competitive: the Republican John McCain.

The defection from Mrs Clinton of Joe Andrew, a former leader of the Democratic National Committee, reflects growing fears within the party that her marathon nomination fight against Mr Obama is helping Mr McCain, who in new polls appears increasingly threatening.

Mr Andrew, appointed the party chairman in 1999 by Bill Clinton near the end of his presidency, said in a letter to other super-delegates: “John McCain, without doing much of anything, is now competitive against both of our remaining candidates. A vote for Hillary Clinton is a vote to continue this process, and a vote to continue this process is a vote that assists John McCain.”

The Arizona senator, who wrapped up the Republican nomination almost two months ago, is running nearly even against both Democrats in head-to-head comparisons, at a time when public approval of the Republican Party is the lowest in a generation.

A race like a crackpot Dynasty

The contest between Barack Obama and Hillary Clinton matches any fiction

Multimedia

• Full coverage: US elections

BACKGROUND

• The ascent of Barack Obama, Mr Charisma

• Decay, jobs and guns fall under the spotlight

• Free trade: Clinton/Obama's mistakes

Related Links

• Super-delegates hold key for Obama

• Obama finally disowns race row pastor

Multimedia

• Blog: the top Hillary Clinton parodies

According to a Wall Street Journal/NBC News poll released yesterday, only 27 per cent of voters have a positive view of the Republican Party, a majority want the Democrats to recapture the White House, and an unprecedented 73 per cent think that the country is on the wrong track.

Yet despite such deep disaffection with the Republican brand, Mr McCain looks an increasingly viable general election candidate. One factor for his competitiveness is his reputation as a maverick who has been willing to buck party orthodoxy; another is that voters identify more with his “background” and “values” than with those of his two Democratic rivals.

Mr McCain's strategists have been heartened by the length and destructive nature of the Democratic race, particularly the controversies that have erupted in the past month around Mr Obama. Until recently they viewed the Illinois senator as a more formidable opponent than Mrs Clinton. Now they believe that he is vulnerable, because of the behaviour and statements of his former pastor, and Mr Obama's own remarks that blue-collar voters “cling” to guns and religion.

Both controversies appear to have taken a significant toll of Mr Obama. New polls show his once formidable, 20-point lead in North Carolina, whose primary is on Tuesday, has been cut to between 5 and 10 per cent. In March, by a 23-point margin, voters had a positive view of the Illinois senator. Now it has been slashed to nine points.

Yet both Democrats have been damaged. Should Mrs Clinton prevail, Mr McCain will face a candidate whose “negative” ratings have been pushed to their highest levels.

Mr McCain has been able to devote precious time to rallying the party behind him, amid signs that he is having significant success. More Republicans now view their party as being as united as Democrats see theirs.

The Arizona senator has been able to switch to general election mode. He campaigned yesterday in Ohio, which will be a critical battleground in November, while Mrs Clinton and Mr Obama are focused on next week's primaries in Indiana and North Carolina - two reliably Republican states in the presidential contest.

Mr McCain, nevertheless, has significant vulnerabilities. Because of the compelling nature of the Democratic race, he is often barely visible, receiving a third of the media exposure of the Democrats. He is 72, vying to become the oldest first-term president in history. He will be outspent heavily by whichever Democrat he faces. His opponents are already claiming that he is running for “George Bush's third term” - and wants to prolong the Iraq war - at a time when Mr Bush's disapproval rating is the highest for any US president.

Asked if he was concerned about being linked to Mr Bush, Mr McCain said: “I really believe the American people will judge me on what I will be as president, and not on anybody else.”

Times Recommends

• Tehran cannot live up to its rhetoric
• The new unicycle that will ‘change the world’
• We cannot wish away Iran’s bomb