Activists from both major parties say they are seeing unprecedented demand for help registering and voting this year from the estimated 9 million Americans who live abroad.
"We have got considerably more voters reaching out to us. We have certainly had more than 4 to 5 times the contacts that we had in 2016," said John Foote, a voting assistant with Republicans Overseas in Thailand.
After a race in 2016 decided by tiny margins in a handful of swing states, Inge Kjemtrup, chair of Democrats Abroad UK, said: "This time no one is complacent."
"I've got people who've lived here for four decades and haven't voted before and are getting in touch to say they want to vote this time."
President Donald Trump has long railed against mail-in ballots, saying they contribute to voter fraud, despite no evidence. Overseas voters, civilian and military, have voted by post in US elections for generations.
US law guarantees Americans living abroad the right to vote in federal elections, with votes counted in the state where they or their parents last lived.
States have strict deadlines to register, get a ballot and send it back, though a Federal Write-in Absentee Ballot can be used by voters who don't receive theirs in time. Both parties have overseas arms that can help with the paperwork.
Voters worried about slow postal systems can drop ballots at US embassies, which the State Department recommends doing by Oct. 1.
DHL DELIVERY
But Katherine Theobalds, a shoe designer living in Buenos Aires, was taking no chances getting her ballot back to New York. She forked out for DHL delivery.
"Most people I know are at the stage where they are concerned about getting their ballot to begin with, let alone sending it back on time."
Melanie Lansing, 57, a health insurance agent in San Miguel de Allende, a popular town for US retirees in Mexico, asked someone heading to the United States to send her ballot to Oregon for her: "I wanted to put it in the hands of a friend."
In past elections, the overseas military vote has leaned Republican, but expatriate civilians in many countries lean towards Democrats. In Paris, Republicans sense the Democrats upping their game after approaching the 2016 election with perhaps too much confidence Hillary Clinton would win.
"They are hyper-active in the sense of getting people out. It wasn't like that in 2016... They didn't feel there was a threat from Trump," said Randy Yaloz, head of Republicans Overseas France.
Kjemtrup of Democrats Abroad UK said there was increasing understanding that the overseas vote is big enough to have an impact.
"In the 2018 mid-terms, one of every 25 races across the nation was won or lost by 1% of the vote or less. That's the margins that Americans abroad bring to every congressional district," she said.
One country where Trump is popular is Israel, home to 250,000-300,000 Israeli Americans, whose votes could affect the outcome in at least one big swing state.
"We have, by my estimation, between 25,000 and 30,000 eligible Florida voters here, and that could be the deciding factor in that state's election," said Marc Zell, head of Republicans Overseas Israel.
COVID-19 has made it harder to run traditional in-person events that parties have relied on to help expatriates with their paperwork, said Danielle Miklos, who chairs the Rome chapter of Democrats Abroad. "COVID has put a stop to everything. People don't want to come out."
Karen Birdsall, an international development consultant living in Berlin, is registered to vote in Pennsylvania, a big swing state that sent its ballots out on Sept. 19.
"That means that the time to receive it, to fill it, to mail it back and have it received in time before Nov. 3 is quite short," she said.