ATLANTA — As the midterm congressional primaries heat up amid fears of Russian hacking, roughly 1 in 5 Americans will be casting ballots on machines that do not produce a paper record of their votes.

That worries voting and cybersecurity experts, who say lack of a hard copy makes it difficult to check results for signs of manipulation.

‘‘In the current system, after the election, if people worry it has been hacked, the best officials can do is say, ‘Trust us,’’’ said Alex Halderman, a voting machine expert who is director of the University of Michigan’s Center for Computer Security and Society.

Georgia, which holds its primary on Tuesday, and four other states exclusively use touchscreen machines that provide no paper records allowing voters to confirm their choices.

Such machines are also used in more than 300 counties in eight other states: Arkansas, Indiana, Kansas, Kentucky, Mississippi, Pennsylvania, Tennessee, and Texas, according to Verified Voting, a nonprofit group focused on ensuring the accuracy of elections.

In all, about 20 percent of registered voters nationwide use machines that produce no paper record.

The focus comes as states gear up for the first nationwide elections since Russian hackers targeted 21 states ahead of the 2016 presidential contest. US intelligence agencies have said that there is no evidence any vote tallies were manipulated but that Russians and others are intent on interfering in American elections again.