In search of 326,695 unreturned ballots, Democrats plan an all-out scavenger hunt in Wisconsin.

They’re well-organized, they’re well-funded, and they have a message: Return your absentee ballot, but don’t use the mail.

The Wisconsin Democratic Party and its supporters had been on a mail-voting education crusade since the coronavirus pandemic hit in March, advising people how to request, fill out and return absentee ballots.

Now, in the wake of a Supreme Court decision Monday disqualifying absentee ballots that are received by election officials after Election Day, the party is imploring voters to return ballots to their election clerk’s office or use drop boxes, rather than putting them in the mail at this late stage.

“We’re phone banking. We’re text banking. We’re friend banking. We’re drawing chalk murals, driving sound trucks through neighborhoods & flying banners over Milwaukee. We’re running ads in every conceivable medium,” Ben Wikler, the party’s chairman, tweeted after the Supreme Court decision.

The party is in search of missing absentee ballots. Of about 1,778,157 Wisconsin voters who requested absentee ballots as of Tuesday afternoon, 1,451,462 have returned them. That means 326,695 ballots are still out there.

The outstanding ballots could make the difference between President Trump and Joseph R. Biden Jr. winning the White House: Though Mr. Biden has maintained a steady polling lead in Wisconsin, Mr. Trump carried the state in 2016 by the razor-thin margin of 22,748 votes.

Wisconsin does not report the party affiliation of voters who request absentee ballots. But in states that do report affiliation, nearly two-third of voters who requested ballots this fall have been Democrats, who polls have found to be more concerned about avoiding polling places amid the pandemic than Republicans are.

Under the Supreme Court ruling, mailed ballots must be received by 8 p.m. on election night or they will not count.

All along, perhaps in anticipation of such a ruling, the party has been advising voters to treat Oct. 20 — not Nov. 3 — as the deadline for voting.

Gladys L. Mitchell-Walthour, a professor of public policy at the University of Wisconsin-Milwaukee, believes the effect of the court’s decision may have been reduced by the educational campaigns by Democrats and others, including the Milwaukee Urban League and Alpha Kappa Alpha, the sorority of Senator Kamala Harris, the Democratic vice-presidential nominee.

“These efforts may lessen the blow of the Supreme Court ruling,” she said.

Even so, Barry C. Burden, a professor of political science at the University of Wisconsin-Madison, said the number of otherwise legitimate votes that will not be counted as a result of the ruling was difficult to predict. “We don’t know what the number will be, but it won’t be zero,” he said.

Wisconsin Democrats have tapped into celebrity connections both to raise money and to promote voter education, including an event hosted last month by the cast of “Parks and Recreation.” (An earlier version of this post misstated that Lady Gaga would be appearing at the event. She is not.)

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