For decades, abortion has been an ideal issue for Republicans. It let them appease their base, bring millions of Catholic and evangelical voters permanently into the fold, and portray Democrats as extremists — all without much political downside. But their success may very well carry the seeds of their ultimate defeat.

The right to abortion has enjoyed solid constitutional protection since the Supreme Court made it legal throughout the land in 1973, a decision it has repeatedly affirmed. An entire “pro-life” movement grew up in reaction. Congress and state legislatures have tried to restrict the right. But the basic liberty recognized by the court has remained almost completely intact.

Its survival, however, has been a reliable political blessing for the GOP. Abortion provided a cause that Republicans could run on year in and year out — and one that overrode all other concerns with a significant segment of the electorate.

Among evangelicals who might have been nauseated by Donald Trump’s unholy lifestyle and predatory treatment of women, abortion was the issue that compelled many to vote for him. Nothing was more important than installing Supreme Court justices who would champion the rights of fetuses.

The beauty of abortion as a political issue is that the efforts to block it didn’t energize “pro-choice” voters nearly as much as it did “pro-life” ones. Many Americans who support legal abortion could vote Republican anyway because they saw no real reason to worry.

With the Supreme Court protecting the right to choose, states had little incentive to pass abortion curbs. When such laws were passed, they got struck down. So “pro-choice” moderates could disregard GOP abortion dogma and vote with other issues in mind.

But the outlook changed last summer, when Justice Anthony Kennedy, who had been the decisive vote on matters of reproductive freedom, announced his retirement. It was “a day that we’ve been waiting for,” Students for Life President Kristan Hawkins told NPR. “Our goal in the pro-life movement has always been to make abortion illegal and unthinkable. So we want Roe to be overturned … and we expect that.”

That prospect grew yet more probable once Trump chose Kennedy’s successor: Brett Kavanaugh, a staunch conservative appeals court judge whose views on abortion rights were not a mystery.

When the Trump administration refused to let an unaccompanied immigrant minor in its custody get an abortion, his appeals court ruled in her favor. But Kavanaugh wrote a dissent, asserting against all evidence that the government’s obstinate obstruction did “not unduly burden the right to abortion.”

Having been confirmed, he is almost sure to provide a vote for rolling back this constitutionally protected freedom. Three conservative justices — Clarence Thomas, Samuel Alito and Neil Gorsuch — would certainly concur. The only question mark is the institutionally cautious Chief Justice John Roberts — but he once wrote a brief calling for Roe v. Wade to be junked.

The state “fetal heartbeat” laws manifest the hope that the court is ready to uphold restrictions that would have been struck down before. State Rep. Terri Collins, sponsor of the Alabama bill, which has no exception for rape or incest, was candid about her purpose. “This bill is about challenging Roe v. Wade and protecting the lives of the unborn,” she said

Tragic as her legislation would be for Alabama women, it should be a political boon for Democrats in 2020. Independent voters and moderates can’t ignore the ominous threat to reproductive freedom. They have to realize that the court may give free rein to legislatures in Alabama and elsewhere — making it imperative to elect abortion-rights supporters at every level.

Extending the Republican hold on the White House, it’s clear, could permanently entomb the constitutional right to abortion, by ensuring a lasting, lopsided conservative majority on the Supreme Court. Anyone who favors legal abortion will have a powerful motive to vote Democratic next year and beyond.

The “heartbeat” bills provide additional motivation. Public support for abortion rights is highest in the earlier months of pregnancy. When the issue was late-term and “partial-birth” abortion, Republicans had a relatively easy time making their case. When they act to prevent early abortions and abortions in the case of rape and incest, though, they are inviting a public backlash.

Republicans have used abortion to succeed at the polls because, for so long, they couldn’t get their way. Once they get their way, the tide is likely to turn.

Steve Chapman blogs at http://www.chicagotribune.com/news/opinion/chapman. Follow him on Twitter @SteveChapman13 or at https://www.facebook.com/stevechapman13. To find out more about Steve Chapmanand read features by other Creators Syndicate writers and cartoonists, visit the Creators Syndicate website at www.creators.com.

IMAGE: Protesters hold signs in front of the U.S. Supreme Court on the morning the court takes up a major abortion case in Washington March 2, 2016. REUTERS/Kevin Lamarque