Thanks to the retirement of one of the nine justices of the U.S. Supreme Court, the abortion issue has just moved back to center stage in this year's campaigns for governor and the Minnesota House.

Not that it's been exactly hiding in the wings, mind you. Since it first appeared in legislative races in Minnesota in 1970 — three years before Roe vs. Wade — abortion has been a regular player in state elections. It got top billing for a long while as the driving force realigning the two political parties, pushing anti-abortion DFLers into Republican ranks and making DFL voters out of pro-choice Republicans.

But that swap eventually became old news. Lately, the issue has been akin to an aging vaudevillian, trotted out at partisan confabs to croon a few familiar bars and take a bow for the party faithful. Rarely in recent years has it been a featured theme in campaign ads or a sparring point in debates.

That just changed, courtesy of the impending retirement of Justice Anthony Kennedy. He may have been an unpredictable swing vote on many matters. But when the issue was abortion, Kennedy was a stalwart defender of a woman's right to choose. A New York Times editorial said that as the court has become more conservative in recent years, Roe vs. Wade was preserved "solely on the strength of Justice Kennedy's vote."

What's his departure got to do with the governor's race and the state House in Minnesota?

Consider this scenario: Kennedy is succeeded yet this year by a justice tapped by President Donald Trump and confirmed by the Republican Senate in large part because of his or her desire to reverse Roe at the earliest opportunity. That opportunity comes in 2019. (OK, that may be a little soon, but it's a possibility.) On a 5-4 vote, Roe is overturned. The question of abortion's legality is left to the states to decide, as they could before 1973.

That would allow the 2020 Legislature and governor to take up the question of whether Minnesota would be among the states that outlaw abortion. In office that year will be the governor and state House elected this fall, plus the Senate elected in 2016. The Senate is now composed of 33 DFLers, 33 Republicans and one vacant seat to be filled in a special election — a seat long held by Lt. Gov. Michelle Fischbach, the daughter and wife of leaders of Minnesota Citizens Concerned for Life.

That's why Sarah Stoesz of Planned Parenthood Minnesota, North Dakota, South Dakota Action Fund has been saying since the Kennedy news broke: "Never has the abortion issue been so front and center as it is for Minnesotans in this election."

That's the word Planned Parenthood and other defenders of abortion rights are keen to spread every day between now and Nov. 6. They say they are confident that if Minnesotans see this election as a referendum on Roe, their side will win. Polls have long shown that a small but persistent majority of Minnesotans favor maintaining the access to legal abortion that they've had for 45 years.

What's more, Stoesz says, her organization's experience fighting anti-abortion ballot measures in South Dakota in 2008 and North Dakota in 2014 shows that even people who say they dislike abortion are loath to make it illegal. In both states, measures to restrict abortion were voted down.

But the word "abortion" isn't on Minnesota's ballot this year. Candidates' names are. This state's progressives aren't much inclined to be "single-issue voters," a civic category they've sometimes scorned.

And abortion talk has been part of the background noise of state politics for so long that some voters are inured to it. They might dismiss the activists' alarms as alarmist.

All of that plus the very size of the Minnesota House — where 132 of 134 seats are contested this year — makes daunting the political task that both of the warring abortion camps face.

But both have been preparing for this moment for decades. Both have extensive volunteer networks and access to funding from national as well as state sources. Both are ready for the abortion fight to move out of the legal arena and into the political one — where some would say it should have been all along.

And both know that what will matter most to the eventual outcome in Minnesota is who wins the governor's race. Stoesz called keeping the governorship in pro-choice hands "paramount."

Prediction: Any voter who can't already recite the abortion positions of the candidates for governor will have that rundown committed to memory before the calendar flips to November.

Until recently, we handicappers held that Minnesota's abortion opponents had the edge in zealotry and ability to turn out single-minded voters. But there are signs that's changing. Planned Parenthood's Lauren Gilchrist said her organization has seen a 10-fold increase in volunteers since the 2016 election. Seventeen months ago, a summons for abortion opponents to protest at Planned Parenthood's St. Paul headquarters drew 10 times as many supporters of the organization.

That day's impressive turnout display is noteworthy because in midterm elections, turnout often tells the tale. Gilchrist says her organization will be reaching out to the most pro-choice segments of the population — young people, single women, women of color — to urge them to vote.

Those have been low-turnout cohorts in past midterm elections. If abortion is to remain safe and legal in Minnesota, their participation rate in this year's midterm election will need to climb.

Lori Sturdevant is a Star Tribune editorial writer and columnist. She is at