Just a day after victory Obama hits his first problem after a former fundraiser is convicted of fraud and bribery

John McCain's Republican backers questioned Barack Obama's fitness to become president last night after a fundraiser of the Democrat nominee was found guilty of corruption.

The conviction of Mr Obama's former friend came only a day after he clinched victory over Hillary Clinton in the Democrat candidacy race.

The Republican National Committee issued a statement within minutes of Antoin Rezko being found guilty by a Chicago court of fraud, attempted bribery and money laundering.

With Mr Obama 'wheezing over the finish line' in the Democrat primaries, the committee seized the opportunity to highlight the ties between the Syrian-born property developer and the man who hopes to become the first black U.S. president.

'This is further proof that Obama's high-flying rhetoric is just that.

'Today's verdict and Obama's friendship with Rezko raise serious questions about whether he has the judgment to serve as president,' the statement said.

Although Mr Obama has cut all ties to Rezko and given any funds the tycoon donated to his campaign to charity, their past friendship could come back to haunt him.

The speed of the Republican assault is likely to set the tone for what promises to be a no-holds-barred five-month battle with former Vietnam prisoner-of-war Mr McCain in the build-up to the November 4 election.

The Illinois senator's 20-year relationship with his preacher, Reverend Jeremiah Wright, almost proved his undoing in the primary campaign after the minister's race hate and anti-American sermons surfaced on the internet.

Controversy: Barack Obama at the U.S. Capitol after a vote on the fiscal 2009 budget resolution last night

Now analysts say Mr McCain, 71, is certain to use 46-year-old Mr Obama's links to a convicted crook as a political weapon.

Rezko, 52, will be sentenced in September after a jury found him guilty of masterminding a multi-million pound scheme to use his political connections to get kickbacks from companies seeking government contracts.

He could face up to 20 years in prison. Although Mr Obama's name was mentioned several times during the trial, he was not accused of any wrongdoing.

Mr Obama has admitted doing some legal work for Rezko in the 1990s and conceded their two families were friends but he insisted he did no political favours. 

Election battle: John McCain could use Obama's links to a convicted crook as ammunition

However, he admitted he was 'boneheaded' in allowing the property dealer to become involved when he bought his Chicago mansion for £800,000 in 2005.

The £125,000 donated by Rezko to Mr Obama's campaign was handed to charity when he was charged.

'This isn't the Tony Rezko I knew,' said Mr Obama yesterday, adding that he was 'saddened' by the verdict.

'Now he has been convicted by a jury on multiple charges that once again shine a spotlight on the need for reform.'

At least one headache was easing for Mr Obama yesterday when Mrs Clinton, 60, sent a letter to supporters saying she will publicly endorse him in a speech tomorrow and pledge to help unite the party.

'The stakes are too high and the task before us too important to do otherwise,' she wrote.