As US President Trump and Democratic nominee Joe Biden battle to turn out new voters, the latest numbers show millions who have not registered are still up for grabs in key battleground states, Fox News reports.

About 13.4 million eligible, unregistered U.S. voters can register and vote on election day in 21 states, according to voter advocacy nonprofit VoteAmerica.

That number represents 34 percent of the total 40 million unregistered, eligible voters in the United States, the organization found.

"When I realized it was so high, someone on my team was like, 'That can't be right,' and double-checked my math," Debra Cleaver, founder and chief executive of VoteAmerica, told Fox News of the 13.4 million figure. "We heard all this doom and gloom this year about voter registration numbers being low, and I thought, 'OK. We'll make up for it on Election Day.'"

Election Day registration is available in Alaska, California, Colorado, Connecticut, District of Columbia, Hawaii, Idaho, Illinois, Iowa, Maine, Maryland, Michigan, Minnesota, Montana, Nevada, New Hampshire, Utah, Vermont, Washington, Wisconsin and Wyoming.

Battleground states with the highest percentage of unregistered, eligible voters who can register on November 3 include Minnesota (16.5 percent), New Hampshire (15.9 percent), Wisconsin (14.7 percent), Iowa (13.7 percent), and Michigan (11.9 percent).

And the process will be relatively easy, Cleaver said. Voters should bring valid forms of identification when they show up to a polling place to register to vote on Election Day. The best form of ID is a driver's license or permit, but voters can also bring a photo ID that shows a name and photograph, as well as some official paperwork that shows a name and current address within the state, like a utility bill or bank statement.

Voters then must fill out voter registration forms, and then they may fill out and submit their ballots.

"It's actually pretty convenient," Cleaver said.

Both Trump and Biden's campaigns have focused on registering and turning out voters who did not cast a ballot in 2016. In Florida, for example, Republicans have netted nearly 200,000 new registered voters since 2016 in a state where registered Democrats once outnumbered registered Republicans two to one.

To find the 13.4 million number, VoteAmerica used the most up-to-date voter file in the United States, which includes a list of all the registered and unregistered voters in the country. Cleaver separated registered and unregistered voters by state, determined which states allow EDR and crunched the numbers to find that 13.4 million eligible, unregistered voters could register and vote on November 3.

"The voter file is a thing that exists. It's put together by companies that advocate statewide data and normalize it, so it was pretty easy to sort people from registered and not registered and then into states that had EDR or not," Cleaver said. "As soon as I realized we were talking about 34% of unregistered voters can still vote tomorrow ... I said, 'Well, team, we are adding an Election Day Registration program,' and everyone jumped on it."

VoteAmerica is using peer-to-peer text messaging to help spread the word and will be sending messages to people who did not necessarily opt in to receiving texts.

"For anyone who lives in a state with Election Day Registration, we mention that. [We say], 'Anyone who isn't registered, let them know that they can register and vote on Election Day.' We're leaning into social media, we have a very big email list that we're blasting, and it turns out...we'd already sent about 100,000 postcards to people in Wisconsin who weren't registered."

Madison, Wisconsin residents Theola Carter, left, and Carrie Braxton fill out their ballots on the first day of the state's in-person absentee voting window for the November 3 election.

13.4m unregistered voters up for grabs in US election