Mitt Romney wants the election to be about the under-performing US economy. Alas however, the main topic on the campaign trail this week has been his business affairs, and yesterday brought more trouble: charges that he stayed on as head of Bain Capital, the private equity group where he made his fortune, for three years beyond his claimed departure date.

The allegations came in yesterday's Boston Globe, which cited documents, including regulatory filings, suggesting that the Republican presidential challenger did not leave the company in 1999 has he has maintained, but remained as chairman and chief executive until 2002, the year he was elected governor of Massachusetts.

The Romney campaign hit back immediately, labelling the Globe report "inaccurate," and insisting that the candidate had "no input on investments or management of [Bain] companies" after 1999. That had been confirmed "multiple times by independent fact-checkers," a Romney spokeswoman said.

True or false however, the claim is bad news for Mr Romney. Not only does it turn attention once more to his times at Bain, which the Obama campaign has turned into an emblem of ruthless vulture capitalism, buying and selling struggling companies for its own gain. It also compounds the murkiness surrounding the candidate's financial affairs, and the lack of detail about his wealth, that includes substantial offshore assets.

Polls show that despite the fuss, Mr Romney is still running more or less level with Barack Obama, less than four months before election day. Even so, the episode feeds into what many Republicans – and not just conservatives still unconvinced by Mr Romney – see as a broader problem for his campaign: that he is allowing himself to be defined by his opponent, rather than aggressively hitting back at President Obama.

For the last week, the headlines have been not about dismal new US unemployment data, but Obama campaign accusations that during his years at Bain Romney was complicit in outsourcing American jobs, that he pays too little in tax, and that he hides his money in places such as the Cayman Islands.

"There is still more about this period and beyond that he doesn't want people to know," said Stephanie Cutter, deputy campaign manager for Mr Obama, after the Globe story was published. "It's time for Mitt Romney to come clean, so that the American people can make their own judgements about his record and his motivations."

After his rough ride on Wednesday at the NAACP meeting in Houston, when he was lustily booed on occasion by members of the venerable civil rights organisation, Mr Romney was on friendlier terrain yesterday as he attended an exclusive fundraiser hosted by Dick Cheney at the former Vice-President's home in Wyoming.