Obama campaign says rival's camp has gone into 'meltdown' after former New Hampshire governor John Sununu is forced to withdraw remarks about the president

Mitt Romney's campaign suffered another setback on Tuesday when a senior member of his team, John Sununu, was forced to apologise for suggesting president Barack Obama is un-American.

Sununu, a former governor of New Hampshire, had earlier resurrected questions about the president's American credentials and attempted to link him to political corruption in his home city of Chicago.

But after widespread criticism he was forced to withdraw the remarks. "Frankly, I made a mistake. I shouldn't have used those words. And I apologise for using those words," Sununu said.

The Romney team fielded Sununu, their national co-chair, in an attempt to reverse ten days in which the campaign has been floundering in the face of an Obama onslaught.

Sununu, during a conference call with reporters and an interview with Fox News, made a wide range of personal jibes in connection with Obama, including a reference to his self-confessed pot-smoking in Hawaii.

"I wish this president would learn how to be an American," Sununu said. The comments sounded like an echo of the claims by some conservative groups that Obama was not born in the US or, having spent part of his life in Indonesia, he is somehow un-American.

The Obama campaign, in response to Sununu, portrayed it as a "meltdown" by the Romney campaign in its efforts to distract attention from questions about the Republican challenger's tax returns and his time at Bain Capital. "The Romney campaign has officially gone off the deep end," an Obama campaign spokeswoman said.

Republicans have been expressing concern in recent days that Romney has failed to respond aggressively enough to relentless Obama attacks over Bain Capital. Having failed to make an attempt to portray Obama as a proponent of "crony capitalism" on Monday, the campaign deployed the outspoken Sununu.

But the tactic backfired when he made the "un-American" jibe, taking an already rancurous few days to a new level. Sununu also attacked Obama over his Chicago background, saying the president had come from the city's "felon" environment where two governors and one of the president's donors, property developer Tony Rezko, had been jailed.

Romney has also been vulnerable over his decision not to release his tax returns beyond those for 2010, which he has already made public, and those for 2011 that he has promised release before the election. A Public Policy Polling survey showed 56% agreed that Romney should release his tax returns and only 34% saying he should not.

The Obama campaign released an ad in Pennsylvania on Tuesday to coincide with Romney's visit to the state, questioning whether Romney paid any tax at all. The ad, titled "Makes You Wonder", said: "Tax havens, offshore accounts, carried interest. Mitt Romney has used every trick in the book. Romney admits that over the last two years he's paid less than 15% in taxes on $43m in income. Makes you wonder if some years he paid any taxes at all."

John McCain, the president's 2008 Republican challenger, was forced to put out a statement denying speculation that he had favoured Sarah Palin as his vice-presidential running mate because his vetting team had seen Romney's tax returns and were worried about their potential impact on the campaign.

McCain accused the media of taking earlier remarks out of context. "I selected Sarah Palin because she was the best fit for my campaign, and my decision had nothing to do with the bogus tax return attacks currently being waged by the Obama campaign," he said in his statement.

A leading political commentator, Charlie Cook, expressed in public the concern many Republicans are expressing in private about the failure of the Romney campaign over the past week or so to match the Obama election machine.

In an article for the National Journal, Cook said it seemed "perverse" that the Romney campaign has not been using ad money to define its candidate and to establish him as a three-dimensional person, rather than concentrating almost exclusively on negative ads about Obama's handling of the economy. This is endangering his election chances, according to Cook, and "has left him open to the inevitable attacks for his work at Bain Capital, on outsourcing, and on his investments. It's all rather inexplicable".

Romney could seize the initiative in the coming weeks with the announcement of his vice-presidential pick, the preparations for the Republican convention on 27 August, the release of July's jobs figures, and his visit next week to London for the Olympics and then to Israel. But being out of the country also carries a risk, potentially leaving the field open to the Obama campaign.

On his trip to London, Romney will hold two fundraising events. He has concentrated much of his efforts since winning the Republican primaries and caucuses on building up a fighting fund for use in the autumn.