A series of last-minute challenges to the eligibility of tens of thousands of mostly Democratic voters by acolytes of Donald Trump risk violating federal law and threaten to complicate vote counting in US midterm elections in key states like Georgia.

Many of those efforts -- including one backed by former Trump National Security Adviser Mike Flynn and Overstock.com Chief Executive Officer Patrick Byrne to toss 37,500 voters from the rolls in a Democratic Atlanta suburb -- have sprung up within the 90-day freeze on voter purges under the 1993 National Voter Registration Act.