Voters in four competitive states will cast ballots in November on electronic machines that leave no paper trail — a lapse that threatens to sow distrust about a presidential election in which supporters of both Donald Trump and Hillary Clinton have raised fears about hackers tampering with the outcome.

The most glaring potential trouble spots include Pennsylvania, where the vast majority of counties still use ATM-style touchscreen voting machines without the paper backups that critics around the country began demanding more than a decade ago. It's also a state where Trump and his supporters have warned that Democrats might "rig" the election to put Clinton in the White House, a claim they could use to attack her legitimacy if she wins.

Similar paperless machines are used heavily in Georgia, where the presidential race appears unusually close, and to a much smaller extent in Virginia and Florida, both of which are phasing them out. Florida has almost entirely abandoned the electronic machines following a number of elections that raised red flags, including a close 2006 congressional race in which Democrats charged that as many as 16,000 votes went missing.