President Barack Obama has taken his rhetorical attacks on Mitt Romney, his Republican challenger, to a new level, calling him a “bullshitter” in a still-to-be-published interview with Rolling Stone magazine.

The comment sent fresh shockwaves through the election race today as Mr Obama wound up a cross-country, 48-hour blitz through seven states.

The President savoured an unsolicited – if not totally unexpected – endorsement from General Colin Powell, a Republican who was Secretary of State to George W Bush.

For days, Mr Obama has been calling Mr Romney untrustworthy because of his sudden transition in the last laps of the campaign from a hard-right conservative to a moderate, giving birth to his “Romnesia” mantra. But the Rolling Stone remark, contained in a brief excerpt of the interview published by Politico, strips the varnish from that view.

It came when an executive editor of the magazine said his six-year-old had asked him to send luck to the President for his re-election. The extract concludes with this piece of reporting: “You know, kids have good instincts,” Obama offered. “They look at the other guy and say, ‘Well, that’s a bullshitter, I can tell’.”

A hoarse Mr Obama was also set tonight to become the first serving President to take part in early voting, casting his ballot in his home town of Chicago.

Getting supporters to take advantage of early voting laws in several swing states has become crucial to the President as he tries to hold on in a down-to-the-wire contest. A new Associated Press-GfK poll showed him trailing Mr Romney nationally by 45 per cent to 47 per cent.

The Obama campaign said it had had no early warning of the Powell endorsement. He telephoned the General to thank him. “We think it sends a strong signal about why he should be sent back for another four years to be commander in chief,” said Jen Psaki, a spokeswoman.