"That is really dirty," said Arnwine, who added that the callers' identities remain a mystery. "It's a very sophisticated operation and it's very widespread, and it's very troubling to us."
The last-minute telephone tactics are only the latest in months of legal and political battles over more restrictive voter ID and other laws, mostly fruitless hunts for supposedly ineligible people on voting rolls in many states and sustained claims that African-American and Hispanic voters are being targeted for intimidation and suppression.
Many of these issues could resurface in the courts after Tuesday, particularly if the presidential race is too close to call or heads for a recount in states such as Ohio or Florida.
"Each of these problems can lead to post-election litigation and gum up the election works," said Wendy Weiser, director of the Democracy Program at New York University's Brennan Center for Justice.
In Florida, where Democrats unsuccessfully tried to extend early voting by an extra day, election officials in most of the state's biggest jurisdictions were accepting in-person absentee ballots on Monday. A chaotic scene Sunday in Miami-Dade County, where the election office opened, closed and then opened again, was not repeated Monday. There were still long lines of people but a much more orderly process.
Other issues that have surfaced include confusion among voters about what kind of identification is required to cast a ballot, particularly in states such as Pennsylvania, where new ID laws were delayed or blocked by protracted legal battles. Arnwine, of the Lawyers Committee for Civil Rights Under Law, said that people must learn the laws affecting voting in their state and not allow themselves to be intimidated by challengers.
"Should an individual come up to you and try to question you, you have to remember you have no obligation to talk to them," she said. "We just want to make sure that people remember to stand your ground, don't be intimidated. Be sure to insist on your right to vote."