In the Most Litigated Election Ever, Early Democratic Wins but Few Clear Signals

lower court, can be appealed in a state where the Supreme Court leans Republican. In Pennsylvania, a federal district judge nominated in 2019 by President Trump is expected to soon hear a Republican challenge to expanded mail balloting, based on the threat of fraud, that parallels the one rejected by the State Supreme Court this week.

That Pennsylvania case, with potentially crucial ramifications for mail-in voting, could race through the appeals process and to the Supreme Court in a matter of weeks, Dr. Hasen said.

The death of Justice Ruth Bader Ginsburg on Friday could complicate things further, removing one of the four liberal votes from the court and increasing the likelihood that conservatives would prevail on appeals that go to the court.

The Michigan and Pennsylvania cases, Dr. Hasen, said, are typical of the biggest advances by Democrats and voting rights advocates — lawsuits in battleground states that seek to liberalize the rules for collecting and counting absentee ballots.

Winning a longer period for counting late-arriving mail ballots — which are being disproportionately sought by Democrats — could prove crucial in states like Michigan and Pennsylvania, which Mr. Trump won by barely 44,000 votes in 2016. In Pennsylvania’s presidential primary election in June, more than 37,000 mail ballots were rejected for a variety of reasons; the volume of mail ballots will be far higher in November.

The prospect of losing votes to third-party candidates, on the other hand, is seen by Democrats as a far smaller worry than four years ago. In 2016, the Green Party presidential candidate, Jill Stein, was branded a spoiler in Wisconsin: Her 31,072 votes in the state were more than the 22,748 that separated Hillary Clinton and Mr. Trump. (Gary Johnson, the Libertarian candidate, received 106,674 votes in Wisconsin.)

This year, however, left-leaning Democrats have lined up behind Mr. Biden. Not having the Green Party on the ballot “changes the win number, but not by much,” said Joe Zepecki, a Democratic strategist in Wisconsin.