The report, America's Overseas Voters: How They Could Decide the US Presidency in 2016, notes that when votes have been tight, expatriate voters have played a decisive role in the outcome of several key political contests in the US - including the 2000 Presidential election.

The report notes that an important player in the 2016 election will be Democrats Abroad, the organization that represents registered Democrat voters among the estimated 6 million US citizens living in other countries.

Democrats Abroad is treated as a ‘state’ for primary purposes and will send a similar number of delegates to the Democrat National Convention as smaller states. The organization is also active in fundraising. Democratic National Committee Chairman Governor Howard Dean will be in London on 23 February to raise funds to help get out the expat vote.

Republicans Overseas does not offer a comparable global primary or formal representation at the party convention for registered Republican expat voters (though Republicans can vote in their home state primaries through absentee ballots, as can Democrats who choose this option rather than participating in the Democratic Global Primary).

It appears that overseas Republican voters differ from those in the United States. A recent Republican Overseas poll showed less support for Donald Trump than in the US. The leading candidate among expat Republicans was Marco Rubio.

The report also considers what the health of the expatriate wings of both parties tells us about the state of the organisations in the US.

Professor Jay Sexton, Director of the Rothermere American Institute said: ‘Both the Democrats and the Republicans should not be complacent about the importance of US overseas voters. For the Democrats in particular, these voters will help to determine both the Clinton vs. Sanders primary on Super Tuesday and the general election in November.’

1 March is 'Super Tuesday', the most significant single day of presidential primaries in the United States. Up for grabs for the Democratic candidates are 13 states and 1,038 delegates (close to one quarter of the total); for the Republicans, 14 states and 689 delegates are in play.

The Democratic Party will hold the first day of its ‘Global Presidential Primary’ on 1 March in polling stations around the world. At stake are 21 delegates (similar in size to the state delegations of Wyoming and Alaska) that will help decide the party’s nominee. A key voting station in the UK will be in Oxford at the Rothermere American Institute (RAI). Bernie Sanders’s brother, Larry, lives in Oxford and will vote at midday on 1 March at the RAI polling station. He will be available for media interviews.

Professor Sexton added: 'But it's not just the primaries where these American expats should be taken seriously. Overseas voting was critical to putting George W. Bush into the White House in 2000, and if things are tight, could be just as important in the 2016 election too.

'As this report shows, candidates – and parties – ignore overseas voters at their peril.'

Polls in the Democrat Global Presidential Primary in Oxford will be open from 12pm to 7pm at the Rothermere American Institute.

As part of the day's activities, the RAI will be hosting a panel discussion entitled 'The Mobilization of Voters Overseas' at 12.30pm. RAI Director Jay Sexton will moderate discussion between Bill Barnard of Democrats Abroad and Stacy Hilliard, former Vice-Chair of Republicans Abroad.