Battle for 250,000 votes is joined in the 51st state of America they call Britain

For John McCain's 2008 London fundraiser it was the plush Spencer House in St James, once the home of Princess Diana's ancestors. Four years on from Mr McCain's defeat, the Republicans' latest attempt to recapture the White House has London's über-wealthy American bankers, fund managers and lawyers heading for Mayfair and a Grade 1 Georgian mansion where the presidential hopeful Mitt Romney will hold a $75,000 a plate fundraising dinner next week on the eve of the Olympics.

With Barack Obama warning that a sitting president will be outspent in a campaign by his opponent for the first time in history, London's expat Democrats have reacted to their party's call-to-arms by shunning Mayfair, at least for the moment, and instead concentrating on the grassroots.

While the Romney bash is now looking to fill the expensive empty chair of Bob Diamond, who pulled out from the high-end fundraiser last week, the Democrats were heading for The Ritz – not the hotel, but the Blue Posts pub round the corner. With the Mayfair locus for Mr Romney's City supporters likely to look straight out of a Conde Nast centre-spread, the fish and chips platter upstairs at the Blue Post looked more miners gala than West Wing glamour.

For Michael Lange, a management consultant who has lived in London for 22 years, the pub gathering ended a Sunday of walking on Oxford St and High Street Kensington carrying a the Stars and Stripes and handing out leaflets reminding his fellow American citizens in London that their vote still counts. "We found an elderly couple in their sixties from West Virginia who didn't realise they were entitled to a postal vote, " he said, sounding like he'd just struck electoral gold in Chelsea.

Lange's effort is part of a Democrats Abroad summer offensive which has already involved a five week, 27 cities, 13 countries, European tour.

Quaide Williams, who drove the DA battle bus from Munich, through Germany, Scandinavia, France, Spain, Holland and elsewhere, said that rather than focus on just money, Democrats Abroad – an official organisation of the party which enjoys its own 'state' status at primaries and conventions – were: "aiming to motivate the base. Over four years things can fall asleep a bit. We aim to wake them up."

Around a quarter of a million Americans based in the UK can vote in November's election. Even without polling, its estimated that 70 per cent lean towards the Democrats.

The "Romney – Believe in America" campaign has hired the upmarket fundraiser ScottPrenn to help focus on those capable of writing big cheques for next week's European fundraiser. And the firm, The Independent has learned, is being unofficially assisted by senior UK Conservative aides helping them connect to London's Republican rich.

Tory headquarters says it doesn't have a problem with party aides being part of the Romney machine this side of the Atlantic but they were less forthcoming about confirming whether Conservative ministers or MPs are meeting the Republican candidate on his Olympic visit.

If Mr Romney loses and Mr Obama remains in the White House for a second term, covert Tory campaigning for Mr Romney will do little for the Cameron-Obama special relationship.

A spokesman told The Independent: "The Republicans are our sister party. We are all part of the international alliance [of centre and centre-right parties] led by John Howard, the former Australian prime minister. And I'm sure Ed Miliband will be holding a fund-raiser for Obama."

It is illegal under US Federal Commission laws for any British citizen to donate money to an American presidential campaign. Those paying to attend and write cheques for Mr Romney in London this month will all need to be US citizens and may be asked to bring passports.

Karin Robinson, vice-chair of Democrats Abroad, isn't surprised at Mr Romney's City focus. "London has a lot of wealth and Romney goes where the money is," although Ms Robinson says her party does do "big" when needed. "In 2008 Michelle Obama came, and we've had some big fundraisers."

However, Conservative party aides and office holders getting directly involved in Mr Romney's campaign is an uneasy development for Ms Robinson. "I would assume that whoever is doing Romney's fundraising, are all US citizens or green card holders. The rules are strict and this does ring a lot of alarm bells."

Republican insiders predict Mr Romney will travel home from London with more than $2 million to help his campaign. The cash is likely to have come from a tight, connected group : employees of Goldman Sachs, HSBC, magic circle attorneys, private equity gurus, and super-rich Americans who simply prefer London to New York.

These Republicans will be unlikely to grace the Blue Posts near the Ritz. Inside the Ritz itself is a better bet.