The 2016 US Presidential Election has been the most expensive ever, costing US$6.6 billion, according to a political financial watchdog.

The figures were released by Open Secrets: Centre for Responsive Politics as America votes in what has also been perhaps the most acrimonious campaign in US history.

It estimates that the 2016 race has cost some US$6.6 billion (NZD$8.9b), about US$86.5 million more than the last presidential cycle, when adjusted for inflation.

It writes that the biggest spikes came in spending from outside money groups, purportedly working independently from candidates. These poured large sums into the races for Congress and the executive office.

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"In total, outside groups have accounted for 26.8 per cent of all spending so far, compared to 24 per cent at the same point in 2012. They have poured in an estimated $1.3 billion, which is $190 million more than in the entire 2012 election."

It found that a 56 per cent majority of that spending came from Republican groups, compared with 37 per cent from Democratic groups.

Some of the largest private donations have included Thomas and Kathryn Steyer, who gave US$57.2 million to liberal causes, Sheldon and Miriam Adelson, who doled out US$47.3 million to GOP-allied forces, and Donald Sussman, who gave at least US$34.4 million to groups helping Democrats.

Facebook co-founder Dustin Moskovitz pledged an additional $35 million to defeat Donald Trump last month.

Open Secrets estimates that Clinton has raised some US$687 million for her campaign and Trump US$307 million.