General Colin Powell, the former Secretary of State, is preparing for a live TV interview tomorrow amid intense speculation that he is ready to endorse Barack Obama’s campaign.

Aides from John McCain’s camp are bracing themselves for another damaging blow, with one being quoted yesterday as suggesting that such an announcement from General Powell would be “personally embarrassing” for the Republican nominee – with whom the General has been friends for 25 years – and would “create momentum against us”.

General Powell, who once considered an attempt to become America’s first black president, has strong credentials on national security policy – one of Mr Obama’s weaker policy areas. Since quitting as Secretary of State in 2005 he has distanced himself from President Bush.

Last month he said that electing a black president would send an electrifying message to the world, before adding that he would base his decision on which candidate “blends a right measure of experience and judgment”.

In London this week, where he attended the Africa Rising festival, General Powell took the stage at the Royal Albert Hall to declare: “I stand before you tonight as an African-American... It took a lot of people struggling to bring me to this point in history.”

Mr McCain’s campaign continues to be dominated by Joe Wurzelbacher, the Ohio plumber who confronted Mr Obama at the weekend to ask the Democrat why he planned to raise taxes for small businesses. The Obama campaign has countered by pointing out that the plumber’s current income level would make him eligible for tax cuts, rather than a proposed increase that applies only to those earning more than $250,000 (£130,000) a year.

At a rally in Miami, Mr McCain said: “Americans ought to be able to ask Senator Obama tough questions without being smeared and targeted with political attacks.”

The Republican also earned laughs at a charity dinner in New York, which he attended with Mr Obama on Thursday night, by announcing that he was replacing his entire campaign team with “a man named Joe the Plumber”. He then suggested that Mr Wurzelbacher’s income would soon be eligible for a tax hike having “signed a very lucrative contract with a wealthy couple to handle all the work on all seven of their houses” – a reference to the McCain family’s bulging property portfolio.

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