John Carter, 71, stood in line for three hours last week to cast his ballot for Jill Karofsky, the liberal candidate for the Wisconsin Supreme Court.

Carter, a retired bus driver from Milwaukee and African American Democrat, said he wanted badly to oust conservative Justice Daniel Kelly. And he wanted to send a message to President Trump and state Republicans, who pushed for in-person voting despite the threat of the novel coronavirus pandemic.

“I saw the line and thought, ‘Oh my God,’ ” Carter said. “’But I have to vote. I must stay here.’ That’s why so many people stood in those lines. They wanted change. They’re looking for a change.”

On Monday, Karofsky claimed a surprisingly decisive victory, defeating the Republican incumbent by 11 points. She was propelled by thousands of motivated Democrats, who were angered by GOP insistence on going forward with the election amid the pandemic — seen by many as a way to suppress turnout and boost the conservative candidate in an obscure state court race.

The mobilization of these voters could signal a warning to Republicans in a state that will be key for Trump in the fall.

“There’s no question in my mind that Democrats are more motivated to vote than Republicans,” said Mark Mellman, a D.C.-based pollster who has done work for Wisconsin Gov. Tony Evers (D).

A number of Republicans said too many factors were at work last Tuesday to draw conclusions about November, notably the presence of a competitive Democratic presidential primary. Scott Walker, a former Republican governor, said in an email that November turnout will be twice as large as it was last week. It’s a “completely different model,” he said.

But some Republicans said the results show a worrisome enthusiasm gap.

“I was surprised at the turnout in the Democratic primary. It was more than what I thought it would be,” said Brandon Scholz, the former chairman of the Wisconsin GOP. The results reminded him how much Democrats hate the president, he said, and what a motivating force that could be.

Although November “will be different,” Scholz said, “a cautionary note should be sounded.”

Democrats are also delighted — and Republicans worried — about the geographic sweep of Karofsky’s victory. She dominated not only in the states’ two liberal strongholds, Milwaukee and Madison, but also in its suburbs and even some of its rural areas.

Charles Franklin, who conducts the Marquette University Law School poll, said the results underscored how changing voting patterns have increasingly cut into Republican margins in suburban counties around Milwaukee.

“Wisconsin has been a little slow to join the suburban shift that other states have seen earlier or bigger, but we are seeing the markers of that,” he said.

Kelly carried suburban Milwaukee counties but by reduced margins from historical norms. “Our suburbs have been deep, deep red,” Franklin said. “They still are red, but by a smaller and smaller margin.”

Meanwhile, Karofsky won small majorities in three counties in the Fox Valley that have long favored Republicans. Franklin attributed the changes there to ever-stronger support for Democrats in the cities of Green Bay, Appleton and Oshkosh, which are surrounded by strong Republican rural areas.

Something else caught Franklin’s eye. Karofsky swept the counties in the state’s southwestern area, which included several that had voted for President Barack Obama in 2012 but shifted to Trump in 2016.

Andrew Hitt, chairman of the Wisconsin Republican Party, said Democratic turnout was much higher than on the Republican side because of the presence of a Democratic presidential primary on the ballot and an uncontested primary for Republicans. He predicted that turnout will be much more even in November.

“Hats off to Democrats for convincing Bernie to stay in the race to help Jill Karofsky,” Hitt said, referring to Sen. Bernie Sanders (I-Vt.), who conceded the Democratic nomination to former vice president Joe Biden the day after the Wisconsin primary. “I don’t know how they did it, but clearly that’s what they did.”

Keith Gilkes, a Republican strategist and longtime Walker adviser, said Trump’s strength in the northern part of the state remained solid, and he senses lack of real enthusiasm for Biden. But he noted that the Trump campaign cannot take for granted support from Republican suburban women and even some Republican suburban men.

“It’s always a good warning for the Republican Party,” he said of the primary results. “I think it’s meant to help keep us on our feet. . . . This isn’t a gimme state and it just fuels our toss-up nature.”

Democrats who accused Republicans of trying to suppress turnout by going forward with the elections in Wisconsin said the GOP should abandon that strategy elsewhere.

“This election took voter suppression to the most dangerous level possible,” said Tom Perez, chairman of the Democratic National Committee. “You were forcing people to choose between their own safety and health and their fundamental right to vote. That is such a cynical ploy.”

And it didn’t work, Perez added. More than 1.5 million Wisconsinites turned out to vote, either in person or by mail — more than any of the spring elections of the past three years.

“The average Wisconsin voter saw that, and saw right through it, and they made their voices heard,” he said.

An additional factor may have been at work last week, and that was an infusion of money from Democratic groups keen on flipping Wisconsin back to their column in November. Among them was the National Democratic Redistricting Committee, the group founded by former Obama attorney general Eric Holder to target Republican governors, legislators and judges who could stand in the way of his fight against partisan gerrymandering of political districts.

Holder’s group gave about $220,000 to support Karofsky’s campaign, including $50,000 to a grass-roots group in Milwaukee called BLOC — Black Leaders Organizing for Communities.

That group’s activity, combined with Republican insistence on going forward with voting even though Milwaukee was able to open five voting locations out of the usual 180, helped spur high turnout, Holder said.

“It’s an indication how you can spend a relatively small amount of money and have an impact, a grass-roots impact,” he said, adding: “Republicans will act at their peril, both the president and people at the state level, if they employ tactics to deny people the opportunity to vote in a way that is safe for them and safe for their communities. I think that will drive turnout up. They will find ways to vote.”

Angela Lang, the founder of BLOC, said one of the group’s great successes was orchestrating a dry run for the fall election, teaching voters how to vote by mail and forging relationships that will last through the cycle. The group made nearly 35,000 calls and sent nearly 82,000 texts in Milwaukee, Lang said.

“People didn’t know that requesting an absentee ballot was an option,” Lang said. “We gave them a crash course in understanding the different ways to vote.”

In the end, about 70 percent of voters cast ballots by mail in last week’s elections — up from a typical number of less than 15 percent.

Franklin, the pollster with Marquette University, offered an additional caveat that makes direct comparisons with November’s presidential election more difficult: Trump himself was not on the ballot, and his ability to draw his voters out is considered one of his strongest assets.

But the patterns that helped decide the Supreme Court race worry Republicans nonetheless. The shifts were evident in the presidential races of 2012 and 2016 as well as in the gubernatorial elections of 2014 and 2018. In all cases, the Republican suburban strongholds were beginning to erode.

Said Gilkes, the GOP strategist: “I don’t think this is going to be an easy win in any way shape or form.”

Simmons reported from Milwaukee. David Weigel contributed to this report.