These legal fights are shaping up to be one of the most important factors in determining whether Trump or Democratic nominee Joe Biden is inaugurated as president in January.

Democrats, backed by some election officials, are pushing to have state deadlines extended due to fears the beleaguered United States Postal Service will struggle to deliver the millions of extra expected ballots on time. Republicans argue, with minimal evidence, that prolonging the counting period will lead to fraud and unnecessarily extend the presidential election.

It’s a fight that could continue in the days, or even weeks, following the Nov. 3 election. The margin of victory in a handful of states is expected to be so razor-thin that late ballots could determine who wins. Even following the election, Democrats will likely push for states to wait for outstanding ballots while Republicans will ask for them to be excluded, arguing, in part, that there’s no way to prove all of the late-arriving ballots were mailed prior to the election because of the lack of a postmark.

“Our concern on some of this is that the rules are unclear and some of the relief the Democrats are asking for in court could realistically allow for ballots to be voted after Election Day,” said Justin Riemer, chief counsel at the Republican National Committee. “Who’s to say the ballot was actually voted on Nov. 3? Reasonable minds can differ about some of these issues, but I think we can all agree ballots shouldn’t be voted after the election.”

A record number of voters — more than half — are expected to cast their ballot by mail this year as Americans avoid the polls during a global pandemic. About 15 million people have already voted early, according to the United States Elections Project, which compiles early voting data.

Already this year, voters have lodged complaints about the delay in receiving ballots from the Postal Service in their state’s primaries. And voting experts caution that returning ballots too close to Election Day could cause problems.

“There are going to be millions of people voting by mail for the first time ever,” said Barry Burden, founding director of the Elections Research Center at the University of Wisconsin, Madison. “They will mostly be unfamiliar with the process. … A lot of voters may not realize how early they need to begin in order to get their ballot in by Election Day.”

Since the spring, Trump has been trying to link mail-in voting to fraud — with scant evidence — as he searched for a strategy to help his reelection prospects amid faltering poll numbers. It’s more likely, however, that human error will lead to problems: Ballots could be sent to the wrong address or get lost in the mail, or voters may have their ballot tossed out by election officials for a variety of reasons — like having a signature or name that doesn’t exactly match the information on file.

While Trump has had no success in stopping the 10 states that send absentee ballots to all voters, he is managing to limit the time frame for counting those ballots when they are returned.

The Trump campaign and Republican National Committee have tried to tighten pandemic-extended deadlines in at least 10 states as part of a broader push to halt or roll back measures intended to expand remote voting during the pandemic. In recent weeks, Republicans have declared victory in half a dozen cases on the ballot deadline issue, even if they have lost on other issues.

Some states have long-standing laws that allow ballots arriving after Election Day to be counted. Ohio, for example, accepts ballots 10 days later if they are postmarked no later than the day before the election. States also accept military and provisional ballots after Election Day.

Republicans argue local election officials and judges are changing these rules too close to an election, noting the Constitution gives state legislatures the authority to regulate federal elections.

“What’s going on in most of these states is that it’s not the state legislatures changing these rules, it’s judges coming in and saying laws set by the state legislature are not good enough,” said Hans von Spakovsky, who manages the conservative Heritage Foundation's Election Law Reform Initiative. “You have judges … suddenly extending the deadline for absentee ballots. Judges can't do that.”

But Sean Morales-Doyle, deputy director of voting rights, elections and democracy at the Brennan Center for Justice, said that in some cases, Republicans are actually challenging deadlines that legislatures have set.

“The particulars of deadlines for mailing and receipt are set by the legislatures in the first instance, but our system is set up to allow the courts to step in as needed to protect and preserve the right to vote,” he said.

To counter Republicans, Biden and the Democrats have launched a $100 million education program using texts, mailers and digital apps focused on explaining to Americans how to vote by mail and in person and pushing them to vote as early as possible, according to a Biden campaign official. About 2,500 staffers across the country are helping in the effort.

There are signs Democrats may be making inroads of their own. Recent polling shows Democrats are more likely to vote by mail and that Democrats are outpacing Republican requests for absentee ballots in some swing states.

"We're confident that the voting process will go smoothly this November, and we've built the largest voter protection program in history to ensure that's the case,” said Biden spokesman Michael Gwin. “Republicans may futilely try to restrict voting access, but Americans will have more options this year on how to cast a ballot than ever before, and we've put in place a massive voter education and organizing program to help them navigate the process.”

Still, Trump’s campaign and the RNC are spending $20 million to challenge moves to expand mail-in voting and lengthen absentee ballot-counting deadlines. They are also recruiting thousands of attorneys to dispute results on Election Day.

Republicans have had successes in some lawsuits over everything from the location and number of ballot drop boxes to whether absentee ballots require witness signatures, in addition to the deadlines for counting remote ballots. Since 2006, the Supreme Court has repeatedly warned courts not to allow significant voting changes close to Election Day, in one case citing 75 days out as a deadline.

The RNC counts wins — at least on the deadline issue — in Wisconsin, New Hampshire, Georgia, Maine, South Carolina and Florida. And in a case the RNC was not even involved with, the Montana Supreme Court overturned an appeals court decision that would have allowed votes to be counted until the Monday after Election Day.

“An Election Day exists for a reason,” said RNC national press secretary Mandi Merritt. “Democrats are diminishing the integrity of the process by trying to force states to count ballots for days, and even weeks, after Election Day. Voters reasonably expect timely results, and moving the goal posts for late-arriving ballots to continue to be counted only dilutes confidence in the outcome.”

Republicans have not won all their challenges to ballot-counting extensions. They’re asking the U.S. Supreme Court to reject the Pennsylvania Supreme Court’s ruling that allowed a three-day extension. In swing-state Michigan, a judge ruled that ballots postmarked before Election Day can be counted if they arrive within two weeks of Election Day. In Nevada, a state Trump is hoping to pick off, a judge ruled the Trump campaign lacked standing to contest a change allowing ballots to be counted as long as they are received up to three days after the election. And in North Carolina, a judge declined on Wednesday to block state election officials’ policy extending the deadline for ballots to Nov. 12. Democrats are also appealing some of the Republican wins, such as in Wisconsin.

But conservatives are anticipating the actual election could give them legal standing to launch further challenges.

“A lot of times when you are doing pre-election litigation you might fail because what you’re alleging is not yet real,” said Jason Snead, executive director at the conservative Honest Elections Project. “Once you have a single rejected ballot or a ballot you say is invalid, then that harm is real and concrete and you can bring some additional litigation.”

Josh Gerstein contributed to this report.