For now, it is mostly up to state and local governments. In Oklahoma, for example, Gov. Mary Fallin vetoed legislation in April that would have allowed voters to take selfies with their ballots. Election officials say the rules are now unclear. California, by contrast, repealed a 125-year-old law in 2016 that barred voters from voluntarily disclosing how they voted. A more selfie-friendly law went into effect there in 2017.

But even critics of stricter rules caution that voters should respect the rights of others who may not want to have their photograph taken. A study published in 2016 by several groups including the Electronic Privacy Information Center, a nonprofit privacy research center in Washington, found that 44 states had a provision in their constitutions that guaranteed secrecy in voting. (The remaining six had a statutory provision.)

In the end, how laws are enforced may depend on turnout and how vigilant poll workers are this year. “If the lines are getting longer because people keep taking pictures of themselves, they will tell them to stop,” said Bobby Hoffman, the advocacy and policy counsel for voting rights at the A.C.L.U. in Washington. And is it likely scores of selfie-takers will find themselves in court? Probably not.

The Democratic nominee for governor in Florida, Andrew Gillum, ran afoul of the state’s no-selfie law when he took a photograph of his ballot during the August primary. “Today I voted for me,” he wrote, filling in the bubble next to his name and posting the photograph on Twitter. Mr. Hoffman laughed when he considered the possible fallout.

“The idea of prosecuting someone for voting for themselves is a bit silly,” he said.