More than 140 of November’s congressional elections will be decided using electronic voting machines with no verifiable paper trail, leading to concerns among election officials that any hacking or tampering will be undetectable—and accurate recounts or audits in the event of extremely close races, difficult to verify.

Nationwide, Reuters reports, 14 of the 40 most competitive seats are in districts where voters will not have access to paper ballots. These races will take place in states that include Pennsylvania, New Jersey, Texas, Florida, Kansas, and Kentucky.

“Voter confidence is a really big thing, and it’s the battle I worry about losing,” Jonathan Marks, Pennsylvania’s elections commissioner, told Reuters.

In addition to undermining voter confidence, the lack of a paper trail could leave the machines vulnerable to election-meddling, officials say. The concerns come as anxiety over the legitimacy of elections is growing, with nearly three-quarters of voters telling CNN earlier this year that they are concerned about foreign interference in the American voting process.

“There’s an urgent need to move away from paperless electronic voting machines, ideally before the fall general election.” —Aquene Freechild, Public Citizen