Americans are expected to vote in record numbers tomorrow in an affluent urban neighbourhood 3,000 miles from Washington on the wrong side of the Atlantic.

A special primary for Democrats living in Britain, many of them mortified by the Bush presidency and haunted by memories of the 2000 election debacle, will be held in a Victorian function room a stone’s throw from Notting Hill.

London’s 200,000 American voters, who include some of the highest-paid figures in the City, have already been courted assiduously for their money by Bill Clinton, Michelle Obama and Rudy Giuliani. Democrats among them are expected to turn out in force tomorrow for the West London walk-in primary largely because of anguish over the damage that their country’s image has suffered since the Iraq invasion. One organiser described President Bush as “our best recruiter by far”.

Unlike Republicans, who will gather tomorrow for a low-key evening in a Covent Garden bar, Democrats abroad can vote for “foreign” delegates as an alternative to voting by post in their home state. If other primaries tomorrow prove inconclusive, these delegates could help to decide between Hillary Clinton and Barack Obama as the party’s nominee.

The Democratic machine is taking no chances, even at this early stage. The “51st primary” is complex, expensive and deadly serious: the London event in the wood-panelled splendour of Porchester Hall is merely the biggest of dozens across the globe, among them Iowa-style caucuses in 34 countries including Russia, Turkey and Ukraine. Ballots will be sent by overnight courier to an international counting centre in Geneva, where the result will be announced later this month, with a total of 22 delegates allotted to the candidates by proportional representation.

Twenty-two is not much. “It’s about half what New Hampshire gets,” says Bill Bernard, a former history professor from Alabama and chairman of Democrats Abroad UK. “We’re right there below Wyoming.”

Expatriate voters’ clout at the convention will be further dilutedbecause their delegates receive only half a vote each. But with polls showing the races tightening between Senators Clinton and Obama in most big Super Tuesday states it is still possible that the Democrats will have to wait for the convention to choose their candidate, in which case delegates chosen by London bankers or Peace Corps workers in Uzbekistan could tip the balance.

It seems that harsh lessons of history may be behind a late surge in online registrations to vote.

In the 2000 presidential race Al Gore lost Florida in the initial count by 527 votes — a margin that the party believes could easily have been erased with better mobilisation of the international vote. In 2004 John Kerry’s campaign thought it had learnt the lesson, only to fail in Ohio in similar circumstances.

It wasn’t until the 2006 mid-term elections that international votes led to Democrat success. Jim Webb unseated a Republican senator in Virginia by barely 2,000 votes — less than the number cast for him abroad.

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