WASHINGTON >> Persistent reports of robocalls incorrectly telling voters they can cast ballots over the phone and fears of aggressive challenges by monitors at polling places threaten to mar Election Day in many key states, voting rights advocates said Monday.

The fake phone calls, some of which involve live callers, continued to crop up in Virginia, North Carolina and Florida, primarily among African-American voters, said Barbara Arnwine of the Lawyers Committee for Civil Rights Under Law. The group has mounted a counteroffensive of tens of thousands of calls reminding voters they can’t cast ballots over the phone.

“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.

Voting-rights advocates pointed with particular concern to the tea party-linked True the Vote organization, which has pledged to dispatch thousands of monitors to polling places to guard against potential voter fraud. True the Vote President Catherine Engelbrecht rejected suggestions that the group would be overly aggressive or issue false challenges.

“Contrary to various interest groups’ statements, True the Vote has never been investigated or charged with intimidating voters,” she said. “A poll watcher’s sole purpose is to monitor the process of our elections. They are trained to never speak with voters, only authorities within the poll.”

The Justice Department will have at least 780 observers at key polling places in 23 states to ensure compliance with the 1965 Voting Rights Act and look into any allegations of voter fraud.

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.

About 200 people were waiting in line at midday Monday at the main Miami-Dade election office. Olga Vila said she only waited about 10 minutes to cast hers.

“I figured the lines were going to be worse tomorrow,” said the 47-year-old accountant. “We Latin people wait until the end. That’s why I’m here today. I should have mailed it a week ago. I could’ve mailed it on Saturday, but then I thought if the post office didn’t do their thing then my vote wouldn’t count.”

Provisional ballots were the latest legal skirmish in the critical battleground state of Ohio, where Secretary of State Jon Husted’s latest decision on how they can be cast was challenged in federal court. Advocates and lawyers for labor unions contend that Husted’s order would lead to some provisional ballots being wrongly rejected because the burden of recording the form of ID used on a provisional ballot is being placed on voters, not poll workers as in the past.

A decision was not expected before Election Day, but the judge overseeing the case planned a ruling before Nov. 17, when provisional ballots can begin to be counted in Ohio. Provisional ballots are used more often in Ohio than most states, with experts predicting between 200,000 and 300,000 will be cast there.

“That could be a huge problem after Election Day for counting ballots,” Weiser said. “There’s really tens of thousands of voters in Ohio whose votes could be at risk.”

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 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.”

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Associated Press writer Suzette Laboy in Miami contributed to this report.