John McCain's campaign has said it has only $2m cash on hand

Their departure, coming a week after a shake-up of his campaign following disappointing fundraising results, is likely to be a blow for Mr McCain.

He has been trailing in polls to former New York Mayor Rudy Giuliani and former Massachusetts Governor Mitt Romney.

In the Senate, Mr McCain defended the US troop build-up in Iraq, the stance analysts say has cost him most support.

Mr McCain said reinforcements were only just in place and should be given more time, echoing the message of President George W Bush and senior US military commanders.

"Make no mistake. Violence in Baghdad remains at unacceptably high levels," he said, speaking after his sixth visit to the country.

But, he contended, the efforts of the US and Iraq were "moving in the right direction".

Staff cuts

Mr McCain's Senate address coincided with the announcement that campaign manager Terry Nelson, a veteran of President George W Bush's 2004 campaign, and chief strategist John Weaver had quit. Longtime aide Rick Davis has taken over as manager.

Following last week's news of Mr McCain's $11.2m fundraising total for the second quarter of 2007, Mr Nelson had said he was working for free to save money.

The senator revealed that he had only $2m cash on hand and dozens of staff reportedly lost their jobs in the ensuing shake-up of his operation.

By comparison, Mr Romney announced a second-quarter fundraising total of $20.5m (£10.2m), of which $14m (£7m) was money raised for primary campaigning and $6.5m was a loan from his personal fortune.

Aides to Mr Giuliani said he had more than $18m cash in hand and no debt.

For the Democrats, Barack Obama reported raising $31m for the primary battle, putting him ahead of Hillary Clinton with $21m for the primary and another $6m for the general election.

Republican Ron Paul, an anti-war congressman from Texas considered an outside chance for the Republican nomination but with loyal online supporters, announced he had $2.4m on hand, having raised less but also spent far less than Mr McCain.