She thinks Barack Obama is a “phenomenon – and obviously an historic figure”. She has “enormous admiration and empathy” for Hillary Clinton.

However, Carly Fiorina, one of America’s most successful businesswomen, is backing John McCain for president and aims to lure disaffected Clinton supporters into his camp.

As the former chief executive of Hewlett-Packard, the computer conglomerate, Fiorina, 54, was routinely hailed as the most powerful woman in American business, until she was unceremoniously dumped as the company floundered in 2005.

Three years later Fiorina has re-emerged in an eye-catching double role as McCain’s chief economic adviser and his most appealing media cheerleader.

Republicans describe Fiorina as the perfect magnet for Clinton supporters who wanted to see a woman president and may still be bitter that Obama beat her to the nomination.

At one point during the primary campaign, as many as 25% of Clinton’s voters said they would not vote for Obama, although mass Democratic defections appear unlikely now that she has endorsed him.

“The point, I think, is that the number of people registered as independents has grown dramatically,” Fiorina said. “Many of those are women and there’s a real opportunity for us to talk to them about John McCain.”

Suddenly Fiorina is everywhere on American television – a tall, glamorous, persuasive surrogate neatly deflecting criticisms of McCain and politely suggesting that Obama’s political record may not quite match his soaring rhetoric.

It has been a measure of Fiorina’s unexpected political effectiveness that the Democrats have begun to make her a prime target of anti-McCain attacks.

“They have been putting out nasty and untrue allegations," she said. “Not just about the senator but also about about me, for heaven’s sake.” Yet she has proved so adept at defending both McCain and herself that she has also been mentioned as a long-shot vice-presidential option, should McCain seek to counter Obama’s celebrity with an unconventional choice of running mate. Many Republicans are convinced she will end up with a high-level job in any McCain administration.

The daughter of a Texas law professor, Fiorina once worked at H-P as a secretary before joining AT&T, the telephone behemoth. She rose through corporate ranks before shattering the “glass ceiling” supposedly holding back female executives.

After leading H-P through a takeover of Compaq, a rival computer firm, she came under heavy fire when her strategy initially seemed to have crippled the merged conglomerate, but her successor stuck with some of her strategies, and the company has begun to recover.

She has turned out to be a television natural, an ambassador for a Republican campaign that has been widely criticised for its lack of sparkle. Yet no sooner did she emerge as a threat than the Democrats began to circulate insulting accounts of Fiorina’s supposed business failures.

“With advisers like this, it’s no wonder John McCain doesn’t understand the economy,” said Karen Finney, the Democratic national committee’s communications chief.

The Democrats also contrasted McCain’s criticisms of excessive executive pay with claims that Fiorina had received a “golden goodbye” pay-off of $42m (£21m) when she left H-P.

Fiorina, a lifelong Republican who admires McCain’s independent streak, says she received about $14m (£7m). Last week she attacked Obama for allowing his campaign to indulge in political “nastiness”.

She added: “It was Obama who promised a different kind of politics. He said, ‘I’m raising the bar, I’m going to play a different kind of game’. But it’s his campaign that’s engaging in politics as usual.”

McCain may be behind in the polls, by anywhere from five to 15 points, but Republicans are convinced that Obama is heading for an embarrassing U-turn on Iraq, and that he will be forced to admit he cannot withdraw troops as quickly as he has previously indicated. “There’s much about Obama to be admired and respected,” Fiorina said carefully. “ Frankly, as an American, I’m proud that an African-American can achieve these heights.”

There just had to be a “but”, and Fiorina duly obliged with characteristic deftness. “But,” she added, “he seems to be a person who’s willing to do and say what’s necessary to win. I guess his campaign has decided that raising the bar high isn’t worth it after all.”

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