ATLANTA (AP) — U.S. Postal Service warnings that it can't guarantee mailed ballots will arrive on time have put a spotlight on the narrow time frames most states allow to request and return those ballots.

The mail-in ballot deadlines are tight in the best of times. But many more ballots are at stake this year with tens of millions of Americans likely to vote by mail because of concerns about coronavirus exposure at polling places.

Georgia voters can request a ballot by mail until four days before the Nov. 3 election, with completed ballots due by 7 p.m. on Election Day. New Jersey ballots mailed on Election Day must be received within 48 hours of polls closing. Louisiana voters who meet certain criteria can request a ballot by mail until the Friday before Election Day, but they must return them by the following Monday — a day before the election.

In letters to state election officials late last month, the Postal Service warned many of the deadlines for absentee ballots and ballot applications "may be incongruous with the Postal Service's delivery standards." The warnings became public last week amid outcry over mail delivery disruptions and concerns that President Donald Trump is trying to undermine the agency ahead of the election.

Ballots received by local election officials after their state's deadline won't be counted. Hoping to avoid that, some states are altering deadlines. Secretaries of state elsewhere, including in Kansas and Tennessee, say it's too late or would be too confusing to make changes.

"The message to voters is really very clear," said Jeanette Senecal with the League of Women Voters. "We need them to make their Election Day plan so they know when they're returning their ballot, how they're returning their ballot (or) if they're voting in person."

People who don't want to vote in person need to make sure they have the most up-to-date information on how to obtain an absentee ballot and how to return it. Rules vary from state to state and even within some states, she said.

A majority of states require mail-in ballots to be received by Election Day, while others accept them days or even weeks later if they're postmarked by Election Day.

The Postal Service letters recommend that requests for mail-in ballots should be received by election officials at least 15 days before the election. Few states have application deadlines that far in advance, and most accept mailed applications within seven days of the election.

Some even allow ballot requests by mail until the day before the election.

A number of states already have made changes, including New Mexico and Maryland, which moved their absentee ballot application deadlines from one week to two weeks before Election Day.

Maryland was responding to concerns by local officials that voters tend to wait until the last minute to request ballot applications.

"We were really worried that if we didn't change the deadline, our voters were going to inadvertently run out of time," said David Garreis, president of the Maryland Association of Election Officials.

Ongoing lawsuits could lead to further rule and deadline changes as the election approaches.

In Minnesota, a lawsuit filed by voting rights groups resulted in the Democratic secretary of state agreeing to extend the deadline for counties to receive mail-in ballots from Election Day to one week later.

Litigation is pending in a number of other states, including Georgia and Wisconsin, where federal lawsuits seek to extend the deadlines for receiving mail-in ballots beyond Election Day to prevent voter disenfranchisement.

Georgia's deadline is set by state law, and Republican Secretary of State Brad Raffensperger's office said a later deadline could interfere with election officials' ability to perform audits and other post-election tasks by certification deadlines.

The judge in the Wisconsin case questioned whether it's appropriate to ease absentee voting regulations, especially if the coronavirus might pose less of a threat to in-person voting by November. But he indicated he might be open to some changes.

In Pennsylvania's June primary, some counties struggled to mail ballots to voters with enough time for them to be returned, and thousands arrived after polls closed.

Democratic Gov. Tom Wolf has asked the Pennsylvania Supreme Court to extend deadlines for mail-in ballots from Election Day to three days later, citing the Postal Service warning. Republicans oppose it.

Other states also are seeing conflicts between state officials and lawmakers over changing ballot deadlines.

In Michigan, Democratic Secretary of State Jocelyn Benson wants to allow mailed ballots to count as long they are postmarked by Election Day and arrive up to two days later. But that proposal has gone nowhere in the Republican-controlled Legislature.

The GOP-led legislature in Ohio has ignored calls from Republican Secretary of State Frank LaRose to change the deadline to request an absentee ballot from three to seven days before the election.

LaRose is urging voters to submit applications no later than Oct. 27, rather than waiting until the Saturday before Election Day.

"Do not wait that long to request your absentee ballot," he said. "The law may permit it, but it is a foolish thing to do and will likely result in you getting a ballot after Election Day."

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Associated Press writers David Eggert in Lansing, Mich.; John Hanna in Topeka, Kan; Steve Karnowski in Minneapolis; Kimberlee Kruesi in Nashville, Tenn; Morgan Lee in Santa Fe, N.M.; Marc Levy in Harrisburg, Pa.; Sean Murphy in Oklahoma City; Todd Richmond in Madison, Wis; Julie Carr Smyth in Columbus, Ohio, and Brian Witte in Annapolis, Md., contributed to this report.