The Supreme Court decision Monday effectively barring the counting of mail-in ballots in Wisconsin that arrive after Election Day was not a surprise for many Democrats, who had pressed for it but expected to lose.

But a concurring opinion by Justice Brett Kavanaugh set off alarms among civil rights and Democratic Party lawyers, who viewed it as giving public support to President Donald Trump’s arguments that any results counted after Nov. 3 could be riddled with fraudulent votes — an assertion unsupported by the history of U.S. elections.

The decision also unnerved Democrats and local election officials in Pennsylvania, where Republicans are asking the Supreme Court to weigh in again on whether the state can accept ballots received up to three days after Election Day. While Democrats in Wisconsin had been appealing for an extension, the current rules in Pennsylvania allow for ballots to arrive three days after the election. Any change could threaten the more than 1.4 million absentee ballots not yet returned.

In his opinion, attached to the 5-3 ruling against the deadline extension in Wisconsin, Kavanaugh wrote that Election Day mail-in deadlines were devised “to avoid the chaos and suspicions of impropriety that can ensue if thousands of absentee ballots flow in after Election Day and potentially flip the results of an election.”

Kavanaugh’s statement mirrored in some ways Trump’s efforts to suggest that only ballots counted by Election Day should decide the result, and more generally to push unfounded claims about widespread voter fraud.

The Wisconsin ruling was the latest in a series of court decisions setting the rules for how voters in different states can cast their ballots during pandemic and when the cutoff is for receiving them.

The ruling revealed a stark divide among the justices in their understanding of the role of the courts in protecting the right to vote during a pandemic.

Both sides expect that Democrats will vote by mail in greater numbers than Republicans will, and that Republicans will vote in person in greater numbers than Democrats will — leading to a potential scenario in which initial results could appear to favor Trump, only to move in the direction of Joe Biden, the Democratic nominee, as the counts of mailed ballots are made public.

Because of a surge in mail-in ballots because of the pandemic, as well as delays at the Postal Service, civil rights groups and Democrats have been pressing for the suspension of certain rules regarding mail-in balloting to ensure that as many ballots as possible arrive on time and so that states and counties will have more time to count them. Republicans have been pressing to keep the more restrictive rules in place.

Kavanaugh’s concurrence was met by a dissent from Justice Elena Kagan, who wrote that “there are no results to ‘flip’ until all valid votes are counted.” “To suggest otherwise,” she added, “especially in these fractious times, is to disserve the electoral process.”