Barack Obama was given stark warning last night of the perils that await him if he becomes the Democratic presidential nominee during a debate against Hillary Clinton in which he was forced to defend his patriotism and links to a violent 1970s US militant.

Helped by the moderators during a prime time debate six days before their next primary contest in Pennsylvania, Mrs Clinton again raised questions about Mr Obama's ability to withstand the Republican attack machine in a general election, central to her argument that she is more electable than her rival.

Mr Obama was forced to defend his recent comments that white small town Americans "cling" to guns and religion, was pressed again on the incendiary remarks of his former pastor, and asked why he does not wear an American flag lapel badge on his suit jacket, like many other US politicians.

Both candidates also appeared to shy away from the prospect of a joint "dream ticket".

"I think very highly of Senator Clinton's record, but I think it is premature at this point to talk about who the vice presidential candidates will be because we're still trying to determine who the nominee will be," Mr Obama said.

Mrs Clinton was similarly noncommittal. "I'm going to do everything I possibly can to make sure that one of us takes the oath of office next January. I think that has to be the overriding goal," she said.

Mrs Clinton, who has suggested in recent weeks not only that Mr Obama would lose to John McCain, but that the Republican nominee-elect was more qualified to be commander-in-chief, was asked if Obama could become president.

"Yes, yes, yes" she said, adding: "Now, I think I can do a better job. That's why I'm here." Yet she went on to build on her argument that Mr Obama is a great risk. She said that she had a lot of "baggage", but that it had been "rummaged" through, suggesting that there was much voters do not know about her rival.

The moderators of the primetime ABC debate asked Mr Obama about his relationship with William Ayers, the rehabilitated but unrepentant member of the Weather Underground, the militant group that bombed the Pentagon and other high-profile government buildings in the early 1970s.

Mr Obama defended himself on all points, but the first half of the debate focused on his vulnerabilities. If he becomes the nominee Democratic and Republican strategists say last night was a mere prelude to what he will face by Republicans in a general election campaign against Mr McCain.

Mr Obama predicted that the Republicans will attack whomever becomes the nominee, but declared that he had shown in his battle against Mrs Clinton that he can "take a punch...I am looking forward to having a debate with John McCain." He added: "It's a debate I am confident I can win."

Asked about his relationship with Mr Ayers, who he met on friendly terms during his Senate run in Chicago in the 1990s, Mr Obama argued against "the notion that somehow as a consequence of me knowing somebody who engaged in detestable acts 40 years ago when I was eight years old somehow reflects on my values."

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