The GOP continues a legal fight against mail ballots that arrive after Election Day

Ahead of this year's election, Republicans have been trying to stop some states — including swing state Nevada — from counting postmarked mail ballots that don't make it to election officials until after Election Day.

That legal effort hasn't been going well. But Republican Party officials say they are committed to these challenges and in recent days they appealed their case in Mississippi to a conservative circuit court in the hopes of getting a favorable ruling there.

Roughly 20 states plus Washington, D.C., accept and count mail-in ballots that are received after Election Day if they are postmarked on or before Election Day. These rules are meant to accommodate voters who don't remember to turn in their ballot until Election Day and to create wiggle room in case there are issues with the Postal Service.

So far the Republican National Committee and others have filed ballot return challenges in Nevada, Illinois, Mississippi and North Dakota.

Early this year, the North Dakota case was thrown out of court. And last month, judges in Mississippi and Nevada also threw out the lawsuits filed by the RNC aiming to disqualify ballots that arrive after Election Day.

The RNC has argued that rules allowing ballots to be counted after Election Day violate federal law. The party argues that Congress — not states — decides when a federal election ends.

Claire Zunk, communications director for the RNC's election integrity unit, told NPR in a statement that "the election should end on Election Day - that's the law, and voters deserve fair and accurate results on November 5th."

She added: "Counting ballots that come in after Election Day in Mississippi and other states threatens election security and undermines transparency for voters."