The 53-year-old tax exile has helped to rescue Conservative finances with vast donations, including £1m in each of the past two years.
A friend of the Thatcher family, he also stepped in to personally guarantee the Tory Party's overdraft at a time when Smith Square was reportedly £3m in the red.
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This largesse has been funded by an immense personal fortune built up through ruthless deal-making, which has often shocked and unsettled the City of London.
Mr Ashcroft has also aroused controversy over his dealings in Belize, from where he has run much of his international financial empire since the early 1990s.
Mr Ashcroft has numerous business interests in the central American country.
He holds dual UK and Belize nationality and is also a "belonger", or citizen, of the Turks and Caicos Islands.
So close are his ties with Belize that he became the former British colony's ambassador to the European Union and more recently its representative at the United Nations.
Political donations
However, some politicians in Belize say his influence does not stop there and that it is far from healthy.
Mr Ashcroft made large donations - rumoured to total about $1m - to the right-wing People's United Party (PUP) when it was in opposition.
Last year the PUP came to power after defeating the centre-left United Democratic Party (UDP). It subsequently introduced several pieces of legislation financially advantageous to Mr Ashcroft.
They included a law giving tax-exempt status to some companies including Mr Ashcroft's offshore holding company Belize Holding Inc (BHI).
The Ashcroft-owned Bank of Belize was also granted the exclusive right to set up offshore companies in Belize for US and UK citizens.
In a recent interview with a British newspaper, Manuel Esquivel, the UDP leader and Belize's former prime minister, urged UK voters to beware.
"What happened to us should be a lesson to you," he said.
Ashcroft rejects 'smears'
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Mr Ashcroft's conduct in connection with the Turks and Caicos Islands, British dependant territory in the Caribbean, has also been criticised.
Leaked Foreign Office memos have revealed that UK diplomats alleged that Mr Ashcroft had threatened to "stir up trouble" for Britain abroad if he was not allowed to open a bank on the islands.
Mr Ashcroft has rejected the allegations. He claimed they were an attempt to "smear" his reputation.
Tory leader William Hague, has also defended the integrity of his treasurer.
He has denied that Mr Ashcroft has ever sought or been offered any concessions in party policy in return for his donations.
Ruthless business genius
Mr Ashcroft is use to being the subject of public controversy.
The self-made millionaire has often hit the headlines because of his aggressive business methods which have have rankled the City's old guard.
However, even his critics would admit he has a genius for making deals.
Mr Ashcroft made his first million at the age of 31 when he sold a cleaning company he had bought with a £15,000 loan five years earlier.
He then bought the Hawley Group, an ailing camping equipment manufacturer which he used as a vehicle to snap up a wide variety of companies.
In 1987 Mr Ashcroft bought the security and motor auctions group ADT for £635m. A decade later he sold it for £2.6bn, personally pocketing about £154m from the deal.
Conservative Party
Foreign Office
Government of Belize
Turks and Caicos Islands
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