Barack Obama has gained an 11th straight victory in the race for the Democratic presidential nomination by winning the Democrats Abroad primary.
Barack Obama has been pushing a message of change
He now faces his rival, New York senator Hillary Clinton, in a TV debate in Texas ahead of crucial primaries there and in Ohio next month.
She is seeking to revive her campaign with wins in the two key states in order to stop Mr Obama's momentum.
Mrs Clinton now needs a majority of the remaining delegates to beat Mr Obama.
He has at least 1,353 - according to an Associated Press projection - of the 2,025 delegates he needs to secure the Democratic nomination at the party's convention in August.
Mrs Clinton has 1,264 delegates.
'Yes we can' message
Mr Obama won 65.6% of the votes cast by more than 20,000 US citizens in 164 countries.
Mrs Clinton polled 32.7%, according to the Democrats Abroad, an organisation sanctioned by the national party.
The Democrats Abroad system of dividing the delegates is unique, and could leave candidates with fractions of delegates.
The primary was used to determine nine pledged delegates, each with half a vote.
Hillary Clinton needs big wins in Texas and Ohio
Due to the system used for rounding up the figures, Mr Obama won 2.5 delegates, while Mrs Clinton won two.
The margin of victory was small but it continues Mr Obama's long-winning streak.
Meanwhile Wisconsin's primary on Tuesday was significant for Mr Obama, says the BBC's Jonathan Beale, because he ate into Mrs Clinton's support base of white women and lower-income workers.
Correspondents say the blue-collar vote will be crucial in the Ohio and Texas contests, and the Clinton campaign has already begun targeting lower-income workers in its ads.
But in his drive to become the first black US president, Mr Obama has gained important support from some powerful unions, including the Teamsters and the Service Employees International Union.
The first-term Illinois senator has brushed off criticism from Mrs Clinton and Republican front-runner John McCain that he lacks substance.
His eloquent speeches and "yes we can" message of hope have inspired many voters.
"It's a choice between a politics that offers more of the same divisions and distractions that didn't work in South Carolina and didn't work in Wisconsin and will not work in Texas," he said to a crowd of 17,000 in Dallas.
"Or a politics of common sense, of common purpose, of shared sacrifice and shared prosperity."
But Mrs Clinton shot back at a New York fund-raiser, saying: "It's about picking a president who relies not just on words but on work, on hard work."
Mrs Clinton has pursued live debates with Mr Obama, hoping her keen grasp of policy issues will be seen to outweigh his sparkling rhetoric.
The Texas debate, at 0100 GMT Friday, is the first of two the Obama camp agreed ahead of the Texas and Ohio primaries on 4 March.
Many analysts say Mrs Clinton can only turn around her campaign with big victories in both states.
She has not won a state primary or caucus since 5 February.
Her husband, former President Bill Clinton, said he thought she could win the nomination over Mr Obama if she won the two large states.
"If she wins in Texas and Ohio, she will win in Pennsylvania and I believe she will win the nomination," he said campaigning for his wife in Texas on Wednesday.
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