If its a federal election, i.e., there is at least one federal candidate on the ballot, the custodian must keep the ballots for 22 months, Brett Kappel, a Washington lawyer with Akerman LLP, said in an email to POLITICO. State law may require a shorter time for retention, but federal law would pre-empt any such state law with regard to ballots cast for federal candidates.
Kappel said evidence in an active court case should never be unilaterally destroyed. He said actual paper ballots are superior to imaged copies, and he pointed to the legal wrangling over Floridas now-discarded punch-card ballots that were banned after the disputed 2000 presidential elections in Florida.
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Asked whether federal law allows for electronic storage and the destruction of federal-election ballots before 22 months, a DOJ spokesman pointed to a federal election handbooks section that says copying is permitted but that the originals be maintained for the requisite twenty-two month period.
https://www.politico.com/states/florida/story/2017/12/15/experts-browards-elections-chief-broke-law-in-destroying-ballots-150258