The number of US millionaires rose by 29 per cent between end-2008 to end-2011 - equivalent to over a thousand new millionaires per day - while HNW wealth is forecast to increase by a quarter by 2016, to hit $23.5 trillion.

While the number of HNW individuals is also expected to grow - by 19 per cent to reach 6.1 million over the same period - "considerable downside risks remain," said WealthInsight, the UK-based global wealth consultancy which made the forecasts.

For example, without political action on fiscal policy in the months ahead, the fiscal cliff could hit HNW individuals in the form of a 5.9 per cent decline in population next year, the firm said. Otherwise, it expects the HNW individual population to rise by 4.3 per cent in 2013.  

While there are (as of 2011) just over 5.1 million millionaires in the US, this still means there are 165,360 fewer than in 2007, the firm said. Collectively, these HNW individuals hold $18.8 trillion - equal to 34 per cent of total individual wealth held in the country. "This is above the global average of 29 per cent, indicating a relatively uneven spread of wealth," said WealthInsight.

Meanwhile, despite experiencing negative growth of -3 per cent, California claims first place in terms of ultra high net worth population, with 4,929. It is followed by: New York (4,363/ 4 per cent growth), Texas (3,828/ 6 per cent growth), Florida (2,597/down 9 per cent), Illinois (1,544/down 17 per cent), Ohio (1,122/7 per cent), Georgia (1,110), Pennsylvania (1,101/down 17 per cent), Connecticut (1,023/26 per cent growth), Michigan (996/12 per cent growth). Of these, eight voted Democrat in this year's presidential election, and Texas is the biggest Republican state for multi-millionaires, according to the findings.

On average, Republican states saw UHNW individual volumes drop by 2 per cent over the review period, while Democrat states logged a 3 per cent decline.

"With signs that growth in the US economy might be slowing, HNW individuals will be worried about the economic uncertainty being created by the so called fiscal cliff," said Christopher Rocks, WealthInsight analyst. "At the moment it’s not entirely clear how either presidential candidate would solve the current political deadlock, which could have a huge impact on America’s wealthy population."

Between 2007 and 2011, the financial services sector was the fourth best performing in terms of the growth of UHNW Americans. During this period, the volume of UHNW individuals within this sector increased by 12 per cent.