Two Minnesota Republicans are challenging Secretary of State Steve Simon's agreement to count mail-in ballots arriving up to a week after the election, adding to a nationwide legal fight over voting rules before the presidential election.

State Rep. Eric Lucero and Ramsey County GOP activist James Carson, both of whom will participate in the Electoral College, filed the suit Tuesday in U.S. District Court alleging that the new deadline violates federal law establishing Nov. 3 as the date of the 2020 election.

Simon, a Democrat, agreed to a later deadline as part of an earlier legal challenge brought by groups seeking to ease mail-in voting requirements out of concern for voter safety during the COVID-19 pandemic.

Under an August consent decree, state election officials agreed to accept mail-in ballots arriving a week late, even if they lack a postmark. The GOP lawsuit alleges that the agreement violates the constitutional requirement that Congress sets the times of presidential elections and that state legislatures set the manner.

"This agreement threatens the integrity of the upcoming election, will result in widespread and severe vote dilution, will (at a minimum) create substantial uncertainty and delay over Minnesota's ability to certify its results," the GOP lawsuit states. They added that agreement "casts in substantial doubt whether the United States Congress will even accept the results of the popular vote in Minnesota."

Simon's office declined to comment on the suit, which is backed by the conservative-leaning Honest Elections Project, a national advocacy group that says it's dedicated to guarding against election fraud. The group has been involved in election-related lawsuits in other states and is airing television ads saying Democrats are changing elections rules to "push a vote-by-mail election that invites chaos and fraud."

Cases contesting absentee ballot and mail-in voting rules have been filed in states across the country as mail-in voting grows in popularity. The legal maneuvering has ratcheted up in intensity as President Donald Trump has questioned the integrity of mail-in ballots, asserting without evidence that they are susceptible to widespread fraud.

Several states, including Washington, Oregon, Colorado and Utah, have used universal mail-in voting for years without significant problems. A now-disbanded presidential commission on voter fraud in the 2016 election became mired in controversy, leading the Government Accountability Office to accept a request by Minnesota U.S. Sen. Amy Klobuchar and others to probe the panel's work, which critics said found no evidence of widespread fraud.

But Trump has asserted that the only way he can lose to Democratic challenger Joe Biden is if the election is "rigged," and warned in a recent tweet that "the Nov 3 election result may NEVER BE ACCURATELY DETERMINED."

Trump allies, including the Faith and Freedom Coalition, have targeted Christian conservatives in Minnesota and other states with mailers warning of "systematic" voter fraud by Democrats, undermining public faith in the outcome of the Nov. 3 election.

Pitted against the GOP's efforts to limit mail-in voting, Democrats and a number of citizen and voting rights groups have pressed to make it easier for people with health risks to vote by mail during the pandemic. Some Democrats also have raised the prospect of post office delays.

In a case brought by the Minnesota Alliance for Retired Americans Educational Fund, Ramsey County District Judge Sara Grewing approved the agreement with Simon to change voting rules, including accepting mail-in ballots received up to one week after the election. While the deal requires that ballots be postmarked by Nov. 3, it also instructs election officials to presume that ballots without postmark dates are sent by Election Day unless a "preponderance of the evidence" shows otherwise.

The GOP lawsuit argues that the consent decree represents a private contract prohibiting Simon from enforcing the law. Honest Elections Project Executive Director Jason Snead said in a statement that the late ballots could threaten election integrity and "force litigation that could drag on for weeks." If Minnesota cannot certify its election results by December, he added, it may be excluded from the Electoral College — "disenfranchising the entire state."

Richard Fiesta, executive director of the national Alliance for Retired Americans, called the Republican challenge "baseless."

He pointed to a U.S. Supreme Court ruling around the Wisconsin primary that allowed elections officials to count absentee ballots postmarked on primary day, April 7, as long as they were received by April 13.

"We see this suit as a threat to the health and safety and the ability of seniors in Minnesota to exercise their right to vote," Fiesta said. He noted that a number of states, including Washington and California, have for years counted votes that arrive after Election Day.

But as recent national polls suggest that about one-third of all voters plan to vote by mail this year, absentee or mail-in balloting has come under increased legal scrutiny, with Democrats and Republicans readying for election challenges both before and after Election Day.