An all-out war over mail voting has erupted in courts across the U.S. Here's what's at stake.

Plenty of these changes are happening in the courts — many state legislatures are out of session — as state officials, advocates and partisans duke it out over how to adapt existing election laws to fit a pandemic.NBC News has reviewed more than 175 suits in 43 states and the District of Columbia contesting everything from whether postage must be prepaid on absentee ballots to where you can vote and how ballots can be legally transported from voters’ hands to election officials. The Post Office has reportedly warned state officials that on-time delivery cannot be relied on for mail elections in 46 states and D.C.,including the 2020 battlegrounds Pennsylvania, North Carolina, Nevada, Michigan, Minnesota, Texas and Arizona.Most states require that election officials receive mailed ballots by a certain time on Election Day, but advocates have sued to get officials to count ballots postmarked on or before Election Day in more than a dozen states.Fights over ballot receipt deadlines are ongoing in key 2020 battlegrounds, including Wisconsin, Pennsylvania and Georgia. It’s going to be hundreds of thousands or millions of ballots that are potentially right on that razor’s edge," Elias told NBC News.The Republican National Committee wants mail ballots to be returned to officials by Election Day, and has intervened on behalf of states in several suits regarding ballot deadlines.And while many states and campaigns will try to counteract the issue of late ballots through voter education — celebrities with major followings like Chrissy Tiegen have also sought to sound the alarm about important deadlines — mail service is not created equal. In Pennsylvania, the Trump campaign is suing to stop the use of drop boxes, saying that allowing these drop points “eviscerates the procedural protections that currently accompany Pennsylvania's vote by mail.”It’s not just drop boxes and ballot collection that has been targeted by Republicans this year, either.The Trump campaign is also suing election officials who seek to help voters request their ballot.