(CNN) The big news out of the Wisconsin primary, which announced its results on Monday, wasn't that Joe Biden had beaten Bernie Sanders in the state. (That was a given.) Instead it was that Jill Karofsky had pulled off an upset of conservative state Supreme Court Justice Dan Kelly . (Yes, Wisconsin elects its state Supreme Court.)

That's a big deal for a bunch of reasons. Wisconsin doesn't usually throw out incumbent Justices; Kelly was only the third sitting state Supreme Court Justice to lose in the past 53 years. President Donald Trump was strongly supportive of Kelly's candidacy. And Wisconsin may well be the single swing-iest state in the country heading into the 2020 presidential election.

So what does it all mean? I asked Milwaukee Journal Sentinel politics and statehouse reporter Patrick Marley that question -- and several more! Our conversation -- conducted via email and lightly edited for flow -- is below.

Cillizza: Give me the lay of the land in this state Supreme Court race before the pandemic hit? Was it considered competitive?

Conservative Justice Dan Kelly was viewed as having a tough road at the start. The Supreme Court race was on the same ballot as the presidential primary, which was expected to draw heavy Democratic turnout. Republican leaders were so worried about this situation that in late 2018 they considered changing the election date so the two contests weren't on the same ballot. They abandoned the plan in the face of opposition from the public and some rank-and-file GOP lawmakers.

But for a time, Kelly's fortunes seemed to improve in his campaign against liberal Dane County Judge Jill Karofsky (and, before the primary, Marquette University law professor Ed Fallone). Kelly was great at fundraising. Conservative Brian Hagedorn won a come-from-behind victory for a seat on the court in 2019 , boosting the morale of Kelly's supporters. In the weeks before the election, the once-crowded Democratic presidential primary was down to just two candidates, which Republicans hoped would dampen Democratic turnout.

As usual, there was no reliable public polling in the race, so observers were going mostly on their guts. Once the pandemic hit Wisconsin and residents were ordered to stay in their homes, neither side knew what to think. Conservatives feared liberals would be better at switching to a campaign that relied heavily on voting by mail. Liberals feared turnout would be curbed as voters focused on the coronavirus outbreak instead of the Supreme Court race.

Cillizza: Did the race change once Gov. Tony Evers issued a stay-at-home order in late March? How so?

Marley: Absolutely. Wisconsin has long had no-excuse absentee voting by mail, but people don't use it much. Instead, those who want to vote early tend to do so in person, either at city hall or, in large communities like Milwaukee and Madison, at libraries and other pop-up voting sites. This time, early in-person voting was not as widely available. Milwaukee, for instance, gave up on it because of fears of spreading the illness.

The situation led to an unprecedented surge in absentee voting . More than a million people voted by mail. That's more than all of the early voting (both in person and by mail) in the 2016 general election. It was a sea change, and it overwhelmed clerks who had to contend with a flood of absentee ballots unlike anything they had seen before.

Cillizza: This sort of thing doesn't happen much; Kelly is only the third incumbent to lose a state Supreme Court seat in Wisconsin in more than five decades. Why did it happen?

Marley: The incumbency advantage in Wisconsin Supreme Court races cannot be overstated. Before Karofsky, just two challengers defeated an incumbent in more than half a century -- in 1967 and 2008.

This year, Karofsky had a baked-in advantage -- the presidential primary. While the Democratic presidential race was virtually over by the time it got to Wisconsin, it still helped drive Democratic turnout. (President Donald Trump had no opponents in the Republican primary on Wisconsin's ballot.)

Cillizza: President Trump tweeted a fulsome endorsement of Kelly on April 3. How much can we extrapolate from this race when it comes to the 2020 presidential race in the state?

Marley: Trump first endorsed Kelly in January at a campaign rally in Milwaukee. This probably motivated both sides. Supreme Court races are technically nonpartisan, but no one really views them that way. Kelly ran his campaign out of an office in the state Republican Party's headquarters. Karofsky got more than $1 million in help from the state Democratic Party.

The Trump endorsement made it even clearer that the race had a partisan dimension. (Kelly wouldn't say who he voted for in 2016; Karofsky said she backed Hillary Clinton and could never vote for Trump.) Also looming in the race was former [GOP] Gov. Scott Walker, who appointed Kelly to the high court in 2016 to fill a vacancy. Karofsky signed the recall petition aimed at removing Walker from office in 2012.

Cillizza: Finish this sentence: "The biggest takeaway from Jill Karofsky's upset is ___________." Now, explain.

Marley: "... Wisconsin remains a purple, unpredictable state."

It's a state where Walker handily won his recall race and a few months later President Barack Obama easily took the state in his re-election bid. Trump won Wisconsin in 2016, becoming the first Republican presidential candidate to do so since Ronald Reagan in 1984. Two years after Trump's win, Democrat Tony Evers defeated Walker and Republicans kept their majorities in the Legislature, ushering in an era of divided government.

What does the latest election mean for this fall? Make predictions at your peril.