Voting rights advocates expressed cautious optimism after the Supreme Court on Monday left intact Pennsylvania’s mail-ballot extension, but their enthusiasm was tempered by concerns over how the court will approach potential future election disputes.

The court’s 4-4 deadlock, which let stand a Pennsylvania court ruling, was a major victory for Democratic nominee Joe Biden, at least for now. Polls have shown Biden’s backers are twice as likely to vote by mail than supporters of President Trump, who won the Keystone State in 2016 by fewer than 45,000 votes.

But in a worrying sign for Democrats and their allies, the Supreme Court’s stalemate broke largely along ideological lines, which are expected to grow more lopsided next week with the seating of Amy Coney Barrett.

The Monday ruling saw Chief Justice John Roberts join the court’s three liberals, while the court’s four most conservative justices — Clarence Thomas, Samuel Alito, Neil Gorsuch and Brett Kavanaugh — indicated that they would have overruled the Pennsylvania court to reinstate a strict Nov. 3 due date for mail ballots.

The tie vote leaves in place the Pennsylvania Supreme Court’s ruling that mail ballots will be counted as long as they are postmarked by Nov. 3 — or have no legible postmark — and received up to three days after Election Day.

Yet even if Roberts and the liberal justices remain united on voting rights issues, which is far from certain, those justices would be outnumbered if Barrett aligns herself with the court’s conservative wing after her likely confirmation next week. That’s left voting rights groups chastened.

Nicole Austin-Hillery, executive director of the U.S. Program at Human Rights Watch, applauded the preservation of Pennsylvania’s mail voting extension but called the conservative justices’ dissenting view “disturbing.”

“[It] portends a future court in which states’ efforts to respect the right to vote may be undermined,” she said.

Some GOP election lawyers expect that Pennsylvania Republicans, who were on the losing side Monday, will renew their bid to have the mail-ballot extension struck down by the Supreme Court after Barrett is confirmed.

During an event with reporters on Tuesday, Republican election attorney Tom Spencer said another Pennsylvania GOP bid would be forthcoming after Barrett takes her seat on the high court.

“I know that with regard to the Pennsylvania decision, the Republicans have filed or are going to file another application when she’s seated because they want to make sure that the decision of that court is overturned,” he said, referring to last month’s decision by the Pennsylvania Supreme Court.

“The big gorilla in the room is the impact of the likely seating of Judge Amy Barrett,” said Spencer, a top official at the Republican-allied group Lawyers Democracy Fund, adding, “It’s going to have an impact.”

Attorneys for the Pennsylvania Republican challengers did not respond to a request for comment.

Barrett would face pressure from both ends of the political spectrum if she is faced with an election-related lawsuit like the Pennsylvania case that could affect how votes are counted in key battleground states. The spotlight would only intensify if the court confronts a case resembling the bitterly divisive Bush v. Gore fight of 2000, which effectively made George W. Bush the 43rd president.

Conservatives hope Barrett represents a reliable vote to overturn court rulings that have eased voting restrictions across the country amid the pandemic and, if necessary, to rule in their favor if a recount dispute reaches the justices.

In courtrooms across the country this cycle, Republicans and their allies have argued that judicial orders to relax voting limits have removed critical election guardrails, opened the vote to fraud and unlawfully taken the management of elections away from state legislatures.

Trump himself has waded into the election law debate mostly by repeated warnings, without basis in fact, of the risk of widespread voter fraud this election. He has also encouraged the swift confirmation of Barrett in part so she could be seated in time to rule on voting litigation.

During the first presidential debate, moderator Chris Wallace of Fox News asked Trump if he was counting on the Supreme Court, including a Justice Barrett, to settle any election disputes.

“Yes, I think I’m counting on them to look at the ballots, definitely,” Trump replied.

Democrats portrayed that scenario as a potential conflict of interest for Barrett and repeatedly pressed her during the Senate confirmation hearings last week to recuse herself from election litigation, which she declined to do. Such calls would be certain to grow louder if Barrett’s participation in any disputes bearing on the 2020 election became an issue.

The Pennsylvania dispute reached the Supreme Court late last month when the Republican Party of Pennsylvania and top members of the state’s GOP-held legislature filed an emergency request asking the justices to block the state’s mail-ballot extension.

Despite Monday’s setback Pennsylvania Republicans can still ask the Supreme Court to rule on the merits of their challenge through a fast-track review process. The extended Nov. 6 mail-ballot due date will stand unless the justices or Pennsylvania state legislature takes further action.

The justices are expected to rule soon on a similar fight involving a Democratic bid to extend the mail-ballot deadline in Wisconsin.