Republican lawsuits challenge mail ballot deadlines. Could they upend voting across the country?

Republicans are challenging extended mail ballot deadlines in at least two states in a legal maneuver that could have widespread implications for mail voting before the presidential election in November.

A lawsuit filed last week in Mississippi follows a similar one last year in North Dakota, both brought in heavily Republican states before conservative federal courts. Democratic and voting rights groups are concerned about the potential impact beyond those two states if a judge rules that deadlines for receiving mailed ballots that stretch past Election Day, Nov. 5, violate federal law.

They...