Ohio got a lot of attention last week for passing an extreme “heartbeat” bill that would ban all abortion after a fetal pulse can be detected — or at around six weeks of pregnancy, which is before many women know they’re pregnant. The bill’s passage on December 6 sparked a flurry of activism, with social media posts urging people to call Gov. John Kasich and tell him to veto the abortion ban.

On Tuesday, Kasich did indeed veto the six-week ban. But he also signed another bill into law that may actually be a bigger threat to the pro-choice movement — a ban on abortion after 20 weeks, with no exceptions for rape, incest, or fetal anomaly, and only very limited exceptions for a woman’s health.

Neither of Kasich’s actions were surprising to reproductive justice advocates. Some speculated that vetoing the six-week ban would help Kasich seem “moderate” on reproductive rights, or distract from the real harms of the 20-week ban.

Is this hateful, ridiculous 6 wk abortion ban just being used as cover for @JohnKasich to quietly sign this hateful, ridiculous 20 wk ban? — Erin Matson (@erintothemax) December 9, 2016

It’s really counterintuitive to think that banning abortion after 20 weeks of pregnancy (when only about 1 percent of abortions occur) could be worse for reproductive rights than banning it after six weeks (which might as well be a total abortion ban, since women would have almost no time to find out they’re pregnant and make an appointment).

But at least in the short term, it’s true. That’s because even if Kasich has let the six-week ban become law, it would have had no chance of surviving the inevitable, immediate court challenge that would follow.

But the 20-week ban might survive in court, and could eventually be taken up by the Supreme Court. And if it did, it would directly challenge Roe v. Wade.

20-week abortion bans are the pro-life movement’s best chance to overturn Roe v. Wade

Ohio isn’t alone; 18 other states have passed similar 20-week abortion bans, which threaten doctors with criminal punishment for violating them. (Three of those laws, in Idaho, Arizona, and Georgia, have been blocked in court because they unconstitutionally limited women’s abortion rights.)

People who support 20-week abortion bans say they’re necessary because fetuses can feel pain at 20 weeks. However, the best available medical evidence on fetal neural development says it’s highly unlikely fetuses can experience pain at 20 weeks.

But there is good evidence that for the pro-life movement, 20-week bans are part of a strategy to overturn Roe v. Wade, the 1973 Supreme Court decision that legalized abortion in America.

“We do think these kinds of cases and these kind of laws will ultimately lead to the erosion and outright reversal of Roe v. Wade,” Ovide Lamontagne, general counsel for Americans United for Life, told TPM’s Tierney Sneed. Americans United for Life has been a leading lobbying force for 20-week bans and other restrictive abortion laws in state legislatures.

When he introduced a national 20-week abortion ban, Sen. Lindsey Graham argued that it would create a “new theory” of when the state has a “compelling interest” to protect fetal life.

In plain English, that means he admitted the law would contradict Roe v. Wade — which also means it could form the basis of an anti-Roe Supreme Court challenge.

At 20 weeks, a fetus isn’t “viable,” or capable of surviving outside the womb. But Roe v. Wade protects a woman’s right to abortion before viability. So if the Supreme Court upheld a 20-week ban, it would dismantle a very basic principle of Roe v. Wade.

A fetus becomes viable at around 24 to 26 weeks. But since every pregnancy is different, viability has to be determined by a doctor. So even if 20-week fetuses could theoretically be viable, banning abortion at that specific point in time would still violate Roe.

(To make things really confusing, some “20-week bans” are written so poorly that they’re actually 22-week bans. A small percentage of infants born at 22 weeks have been known to survive, but not enough that doctors would generally deem a 22-week pregnancy “viable.”)

Abortion opponents do argue that advancing medical technology might push the “viability” line further back in pregnancy. But if “fetal pain” or something like it actually replaced viability as the basic constitutional standard for women’s abortion rights, Roe v. Wade would be pretty much done for.

This strategy is a long shot — but it has a very real chance

To be clear, the Court probably wouldn’t overturn Roe v. Wade unless and until Donald Trump appoints two or three new, ultra-conservative justices.

Meanwhile earlier this year, pro-choice advocates just won a sweeping Supreme Court victory over restrictive state laws in Texas that closed abortion clinics in droves. The five-vote pro-choice majority in that case will still be there, even after Trump appoints a justice to replace the late Antonin Scalia. And the power of precedent in the Supreme Court is strong — so strong that even conservative Chief Justice John Roberts would probably be reluctant to overturn Roe outright.

Roe v. Wade has been weakened before, however — and Anthony Kennedy, the crucial fifth vote on the Texas case, was partly responsible. So advocates have good reason to fear that the same thing could happen again.

In 1992’s Planned Parenthood v. Casey, Kennedy co-wrote the opinion that weakened Roe v. Wade. Originally, Roe said that states can’t regulate abortion at all before the third trimester. But after Casey, states could regulate abortion whenever they wanted — as long as those regulations didn’t create an “undue burden” on women seeking an abortion.

The “undue burden” standard has always been pretty vague, which is why it opened the door for some states — like Texas — to almost regulate abortion out of existence. The Court’s latest ruling pushed back hard against that trend, and strongly reaffirmed abortion rights.

But that pro-choice victory still doesn’t prevent the Court from changing its mind on the basic question of viability — or reevaluating the very backbone of Roe.

So far, at least, 20-week bans haven’t been a winner at the Supreme Court. The Court has refused to hear appeals on 20-week abortion bans in Arizona and Idaho, both of which were struck down by appeals courts.

But if a 20-week ban were upheld by a different appeals court — like, say, Ohio’s Sixth Circuit — that would create a “circuit split,” and the Supreme Court would be forced to issue a ruling.

That’s one reason these laws are still on the books in 15 states, Lisa Wurm, policy manager for the American Civil Liberties Union (ACLU) of Ohio, explained in an interview with Vox.

In some states, advocates haven’t bothered to challenge 20-week bans because nobody in that state offers later abortions to begin with.

In general, though, advocates have to be very strategic about which laws they decide to challenge in court. They have to make sure that if one particular 20-week ban gets appealed all the way up to the Supreme Court, they will have the strongest possible case to strike it, and all similar bans, down.

That’s why there’s no guarantee that Ohio’s 20-week ban will be blocked in court in the near future.

Most people don’t understand why a 20-week abortion ban would hurt women

The danger of 20-week abortion bans to reproductive rights isn’t just constitutional or theoretical. Twenty-week bans do real harm to real pregnant people — but those harms aren’t well understood by the public.

To many Americans, it’s hard to imagine why a woman would need four or five months to decide whether to keep a pregnancy. But most women don’t seek later abortions because they’re indecisive. Some women discover a medical problem late in a wanted pregnancy. Others want an earlier abortion but face long delays due to barriers like high cost, long wait times, or the difficulty of dealing with an abusive partner.

Most 20-week bans, including Ohio’s, don’t make exceptions for fetal anomalies or other problems — even though many serious issues can’t be detected until a 20-week ultrasound.

That’s what happened to Taylor Mahaffey, who was forced to give birth to a stillborn baby due to the 20-week abortion ban in Texas. Rather than induce labor immediately once doctors knew the baby had no chance of surviving, which would have technically been an abortion, Mahaffey had to wait four days for her baby to die and her water to break.

“We cried ourselves to sleep, waiting for him to come,” Mahaffey’s husband Daniel told the Daily Beast. “Eventually she was just screaming at them to get the child out of her.”

Ohio’s 20-week ban doesn’t make exceptions for rape or incest, either, which means a woman who is traumatized by rape would be forced to carry her rapist’s baby to term if she took too long to realize she’s pregnant and process what happened to her.

And Ohio’s health exception is much narrower than Roe v. Wade requires. For instance, if a woman needs an abortion for medical reasons because she has diabetes or multiple sclerosis, she won’t actually be allowed to get the abortion until she is literally sick enough to die or have organ failure.

But most Americans don’t know any of this. Most Americans are not only deeply ambivalent about the morality of abortion; they’re also largely ignorant about how it works and why women seek it.

That ignorance creates some wishy-washy, easily swayed opinions about abortion, polls show. Yes, Americans are much less supportive of legal abortion in the second or third trimester than in the first — but they also don’t want to see Roe v. Wade overturned, which in practice means upholding a woman’s right to an abortion until roughly the third trimester. And Americans are much more likely to support legal abortion after 20 weeks once they understand the reasons women might need it.

In general, people are much more supportive of abortion rights when they think about how women are affected, Vox polling conducted by the communications firm Perry/Undem has found.

But most people either don’t know or don’t always think about how abortion affects real women.

That’s certainly true for one Republican lawmaker in Ohio, who admitted that he’d “never even thought about” why a woman might want an abortion before pushing for the six-week heartbeat ban.

And it’s why many people might praise Kasich for being “moderate” on reproductive rights for signing the 20-week ban and vetoing the six-week ban — even though Kasich’s record on reproductive rights really isn’t moderate at all.

Half of Ohio’s abortion clinics have closed since Kasich took office. And it doesn’t say much about his views if he vetoes Ohio’s heartbeat bill; even Ohio Right to Life thinks the bill goes too far, and it was pushed by an anti-abortion extremist whose views were once too much even for Christian radio.

And 20-week bans like the one Kasich signed could help set off a series of events that completely unravels Roe v. Wade, and ends legal abortion in America.

20-week bans are dangerous to abortion rights because many people see them as reasonable restrictions

The ACLU definitely would have challenged Ohio’s six-week ban if it passed, Wurm said — but again, the organization can’t say the same for the 20-week ban without closer evaluation.

Under Roe v. Wade, a 20-week ban should be just as unconstitutional as a six-week ban. But to many Americans, banning abortion after 20 weeks seems much less extreme, and thus more acceptable, than banning it in the first trimester.

That’s one reason 20-week bans have already passed in so many states, and why advocates still haven’t challenged most of them. Challenging a later abortion ban is simply riskier than challenging a six- or eight- or 12-week ban. Politically, later bans are less unpopular. And legally, some judges share the general public’s biases or misconceptions about later abortion, and will rule accordingly.

Anti-abortion lawmakers and activists have spent years arguing that 20-week bans are reasonable, commonsense restrictions — and the public’s ambivalent attitudes toward abortion make their job a lot easier.

Ohio’s 20-week ban is probably never going to inspire as much outrage or as many memes as Ohio’s heartbeat ban did. And that’s why the 20-week ban could be much more consequential for reproductive rights.