Estimates of exactly how many American citizens live overseas vary widely, but most fall somewhere between 3 million and 9 million. The number who are also eligible to vote in U.S. elections is even harder to pin down.

Still, the online activist network Avaaz thinks they could swing the presidential election.

The group released an online tool Wednesday that aims to streamline the voter registration process for those living abroad, as well as remind them to cast their absentee ballots before the stated deadline. The progressive group hopes that encouraging the American diaspora to vote will help accomplish its decidedly partisan goal: "Defeat Donald Trump."

"For the whole world, whether we like it or not, the United States is a global superpower, which means it can lead us to a brighter future … or it can undo progress," said Emma Ruby-Sachs, Avaaz's deputy director. "People who either live in the United States or are American citizens have a role to play in that and have a responsibility to play that role fully."

Ruby-Sachs said Avaaz members decided to launch the voter registration initiative because of Trump's "seeming lack of regard for some of the value norms we all subscribe to." She specifically cited statements Trump has made indicating he would be willing to use nuclear weapons.

Ruby-Sachs called Americans living abroad a "more worldly group of people, who understand that the United States is part of a global community and want to be a good citizen of that global community." In short, she doesn't expect them to vote Trump.

Of course, in U.S. politics the popular vote is largely symbolic. Who is ultimately named the victor in November will depend on which candidate secures an absolute majority of at least 270 votes from the Electoral College.

But Ruby-Sachs said many Americans living abroad hail from swing states, particularly Florida, where just a few hundred or thousand ballots can determine the winner. In 2012, most ballots sent to overseas voters came from election boards in Texas, California, Florida and Washington state, according to the Election Assistance Commission.

"You don’t have to move a lot of people to have a huge impact," Ruby-Sachs said.

The number of Americans living abroad is poorly tracked. The State Department's Bureau of Consular Affairs offers the federal government's only real prediction, with a rough estimate of 9 million. A report due to be released this month from the Defense Department's Federal Voter Assistance Program puts the number of eligible voters abroad at 2.6 million. Associations representing Americans living overseas say they've spent years trying to get precise figures.

For its part, Avaaz cites an Oxford University study that estimates roughly 5 million eligible voters live abroad, and that only about 12 percent of them voted in the 2012 presidential election.

Currently, the State Department advises U.S. citizens abroad to register to vote in either the state where they most recently lived or one of more than 35 states (and the District of Columbia) that will register citizens who have never lived in the United States. Those include politically contested states, such as Ohio, Colorado, North Carolina and Virginia. Those citizens must request electronic absentee ballots each year, which they then print and mail back to their state's election commission.

Ruby-Sachs said the process is not user-friendly, which likely contributes to low voter participation. So that voters don't have to complete a long government form, Avaaz has created what it calls a "dead-simple tool." It breaks the document down into questions that prospective voters answer. It then populates a completed voter registration form.

Many states require absentee voters to register before an October deadline, and that time frame prompts Avaaz to call its initiative "the October surprise that will defeat Trump." The Avaaz tool will also send regular reminders about deadlines to register and cast a ballot so that voters don't simply overlook them.

Founded in 2007, Avaaz has launched online campaigns for causes that range from ending fossil fuels to maintaining an open Internet to recognizing Palestinian statehood. The group boasts 43 million members in 194 countries around the globe.

Steven Overly anchors and edits The Washington Post's Innovations section. Follow him on Twitter: @StevenOverly. Read more from Innovations