Obama told to "learn to be an American" The vitriolic attacks between Barack Obama and Mitt Romney reached a new low after a supporter of the Republican presidential contender said the US president needed to "learn to be an American." The remark, by former New Hampshire governor John Sununu, came hours after the Obama campaign released an advertisement that said Mr Romney used "every trick in the book" to avoid paying taxes. Against the backdrop of a shot of a Caribbean beach, meant to symbolise Mr Romney's investment in the Cayman Islands, the ad questioned whether Mr Romney "paid any taxes at all." The ad, which was released in Pennsylvania to coincide with a visit to the swing state by Mr Romney, seemed to be the last straw for the Republican campaign, which has struggled with relentless needling by Democrats and even some high profile Republicans on Mr Romney's tax records. The founder of Bain Capital, the private equity group, has released tax returns only from 2010 and an estimate for 2011, although the standard practice for presidential candidates is to release many more. On a call with reporters, Mr Sununu repeatedly suggested that Mr Obama "doesn't understand how America works," a response to the president's stump speech rhetoric about the government's role in helping businesses and entrepreneurs prosper. Earlier in the day, Mr Sununu said on Fox News: "He has no idea how the American system functions. And we shouldn't be surprised about that, because he spent his early years in Hawaii smoking something [and] spent another set of years in Indonesia. Mr Sununu was referencing Mr Obama's teenage marijuana use, a fact that the president has acknowledged and was detailed in a recent biography. The Obama campaign responded with apparent incredulity. "The Romney campaign has officially gone off the deep end," the campaign said in a statement. This meltdown and over-the-top rhetoric won't make things better - it only calls attention to how desperate they are to change the conversation. At the centre of the toxic back-and-forth are two questions about Mr Romney's background. The first is whether the former governor of Massachusetts is trying to hide politically damaging information about his tax avoidance strategy. The second is whether Mr Romney had any managerial oversight of Bain Capital between 1999 and 2002 when the company was linked to outsourcing US jobs. In keeping a sharp focus on these questions, the Obama team has tried to shift attention from the president's own tenure and record on the US economy. Some top Republican strategists have said that Mr Romney may have no other choice but to release the tax records and take the temporary blow of whatever unsavoury financial details may lie within them in order to move the conversation. He needs to get back to the real issues of the campaign, which is about the economy. Until he does [releases more records], it is going to be a challenge," said one. Tom Davis, a former Republican congressman from Virginia, said the issue could resonate through to November, when the Obama campaign is likely to pivot its attacks on Mr Romney to focus on the candidate's support of Medicare reform, a tactic that could inspire fear among elderly voters. Mr Davis said that portraying Mr Romney as an "insensitive rich guy, hiding things offshore" lends "credibility" to other attacks.