Mitt Romney received an embarrassingly rough reception from members of the NAACP, the country's oldest black civil rights organisation yesterday, which erupted into boos and shouts of disapproval when he said he meant to overturn Barack Obama's healthcare reforms.

His rocky ride at the NAACP convention in Houston came as he faced a wave of new assaults from the campaign of President Obama over his apparent failure fully to disclose the extent of and the reasons for his multiple holdings overseas, including now or in the past in Switzerland and the Cayman Islands.

The murkiness of his personal financial affairs is emerging as a potential drag for Mr Romney who so far has released tax returns for only 2010 and an estimate for 2011. Democrats are contrasting him with his father, George Romney, who, when he ran for the White House in 1967, released 12 years of returns.

It prompted an unusually personal attack this week from Vice President Joe Biden who said on a campaign visit to Nevada that the Republican nominee was "making a lie of the old adage, like father, like son". With many Latinos in his audience, Mr Biden made an additional dig at Mr Romney's barely permissive views on immigration, saying that he, "wants you to show your papers, but he won't show us his".

Trying to woo delegates at the NAACP meeting, Mr Romney insisted that his economic vision of free enterprise and reduced spending would lift all families. "If you want a president who will make things better in the African-American community, you are looking at him. You take a look," he pleaded. But even showing up might have been misguided given the overwhelming support among African-Americans for Mr Obama, who took 95 per cent of their vote in 2008. His appearance jumped the tracks completely when he uttered, "I will kill every expensive and unnecessary programme I can find, and that includes Obamacare." With fixed grin, Mr Romney then stayed silent as a wave of boos and shouts of "No!" swept through the hall. The rest of his speech was punctuated by murmurs of dissent.

The tax filings released during the primaries last winter forced Mr Romney to revise earlier financial disclosures, which had made no mention of some 20 newly revealed overseas holdings – a fact highlighted in a harsh New York Times editorial yesterday: "What information he did release provides a fuzzy glimpse at a concerted effort to park much of his wealth in overseas tax shelters, suggesting a widespread pattern of tax avoidance unlike that of any previous candidate," it said. "Neither the tax return nor other disclosures have revealed the full amounts of the Romneys' other offshore holdings over the years, including investments in Germany, Luxembourg, the Cayman Islands, Australia and Ireland," the newspaper asserted.

Questions linger over how much money Mr Romney continues to receive from the Cayman holdings of his former company, Bain Capital. The filings did show that a blind trust held by his wife Ann included a Swiss account worth $3m that was closed in 2010.

So far Mr Romney has countered only with declarations of innocence. "I have followed the law," he told Fox radio host Sean Hannity on Tuesday.

"I have paid my taxes as due. I have also disclosed through all of the requirements of the government, every asset which I own, fairly and honestly, recognising, of course, not to do so would be not only wrong but illegal and criminal."

Chip off the old block? No, his father divulged his wealth…

Mitt Romney

Wealth Lots. As much as $250m according to some.

How he made it Co-founded private equity firm Bain Capital.

Politics Elected governor of Massachusetts in 2002. Lost out on the Republican presidential nomination in 2008 before finally clinching the prize this year.

Controversy His wealth. Unlike his father who released 12 years of tax returns, Mitt's campaign has only released a return for 2010 and an estimate for 2011. The Democrats are asking for clarity on his overseas holdings.

George Romney

Wealth Lots. In a good year at American Motors he made as much as $660,000. Adjusted for inflation that would be worth around $5m today.

How he made it Car industry chief executive.

Politics Elected governor of Michigan in 1962. Went on to run for the Republican presidential nomination in 1968.