Last week, Congress heard testimony on a bill called the "Prenatal Nondiscrimination Act" (PRENDA). It would prohibit a woman from seeking an abortion based on the race or sex of her fetus.

Pro-choice advocates say that bills like PRENDA are based on racist stereotypes that are not only offensive, but also untrue. There's no good evidence that, for instance, Asian-American women decide to abort female fetuses at abnormally high rates because they're trying to have a son.

But Rep. Trent Franks (R-AZ) actually defended the bill on racial-justice grounds:

Throughout America’s history, we have struggled to fulfill that conviction in our national life. It took a civil war in this nation to make the 7,000-year-old state-sanctioned practice of human slavery come to an end, and, ultimately, it did so across the world. American women overcame the mindless policy that deprived them of the right to vote in America. Then this nation charged into Europe and arrested the hellish Nazi Holocaust. We crushed the Ku Klux Klan and prevailed in the dark days of our own civil rights struggle.

And, in so many ways, we have made great progress in the area of civil rights in this country. But there is one glaring exception. We have overlooked unborn children and that life itself is the most foundational of all civil rights.

The result is that today in America between 40 and 50 percent of all African American babies, virtually one in two, are killed before they are born ... When you add to that the thousands of little girls who have been aborted in America simply because they are little girls instead of little boys, these are travesties that should assault the mind and conscience of every American.

Franks is far from the only anti-abortion politician who talks this way. Last year, Ben Carson made headlines for comparing abortion to slavery, and women who have abortions to slaveowners, while he was running for president:

During slavery — and I know that's one of those words you're not supposed to say, but I'm saying it — during slavery, a lot of the slave owners thought that they had the right to do whatever they wanted to that slave. Anything that they chose to do. And, you know, what if the abolitionist had said, you know, 'I don't believe in slavery. I think it's wrong. But you guys do whatever you want to do'? Where would we be?

Many observers found Carson's remarks on abortion outrageous at the time. Some had trouble understanding how Carson could be leading in the polls (which, remember, he was for a while) after saying something so shocking.

But this idea — that fighting against abortion rights is similar to the abolitionist cause — is an incredibly common one in the pro-life movement.

"This kind of rhetoric is actually quite widespread in the movement, and it has been for some time," says Ziad Munson, a sociologist at Lehigh University who studies the pro-life movement. "I haven’t found a period in the movement where that kind of rhetoric was not being used."

Anti-abortion advocates compare abortion to slavery all the time, and Republicans often follow suit

David French argues in the National Review that Carson articulated "an entirely mainstream, pro-life view" that "most Republican politicians dare not utter." That is, that abortion is just as abhorrent as slavery, because both treat an entire class of people (black people in one case; in the other, "the unborn") as less than human.

"In both instances," French writes, "the abhorrent practice rested on dehumanization: the declaration that black Africans were somehow innately inferior to whites, and the declaration that unborn children are somehow less than fully and completely human."

The pro-life movement sees itself as championing the cause of social justice — as fighting for the rights of a voiceless, marginalized group of people. So they often appropriate the rhetoric of civil rights and related causes. Carol Tobias, president of the National Right to Life Committee, has compared the 40-year struggle of the pro-life movement to that of William Wilberforce, who spoke out against slavery in Britain for 40 years before the practice was finally banned. In a 2010 op-ed, Charmaine Yoest, president of Americans United for Life, invoked both the abolition of slavery and the civil rights movement.

Yoest also mentioned the Supreme Court's 1857 Dred Scott decision, which basically upheld slavery. Dred Scott is a popular comparison among anti-abortion advocates; they say that it's just as unjust a ruling as Roe v. Wade, and they hope that someday Roe will also be overturned.

This kind of rhetoric tends to trickle up from the anti-abortion grassroots to more mainstream Republican candidates. George W. Bush once confused pundits by mentioning Dred Scott during a presidential debate, but abortion opponents understood the Roe v. Wade reference loud and clear. Mike Huckabee put his foot in his mouth by suggesting that Dred Scott is actually still the law of the land, but that we still ended slavery nevertheless — which may have been subconscious wishful thinking on Huckabee's part about how to subvert Roe.

These kinds of comparisons almost always cause backlash. So why does it keep happening?

"It’s not always clear to me they think there’s anything wrong with it," Munson said. "It comes as a surprise to them. I don't think they are misspeaking — they are saying things they think are completely unproblematic."

Comparing abortion to slavery has some really awkward historical implications

Pro-choice advocates, of course, oppose comparing slavery to abortion. The real "dehumanization," they argue, is denying women the bodily autonomy that abortion rights allow. But more than that, they say, the comparison is wildly offensive because it ignores the brutal history of reproductive coercion in slavery.

"If you think about it, the claim that abortion is like slavery is exactly backwards," wrote Imani Gandy at RH Reality Check. "I’m not a fan of comparing anything to slavery that is not slavery, but I’m fairly certain that we can all agree that slaveowners systematically forced Black women to give birth."

Slaveowners often used enslaved women as breeding stock to produce new future slaves. Women were raped by their masters and forced to bear the resulting children. They had no rights to their own offspring, much less their own bodies — they were sold or beaten if they couldn't or wouldn't reproduce, and if they did have children there was no guarantee they'd ever get to raise them. For black women forced to live as slaves, home remedies for contraception or abortion became forms of self-defense and resistance, and assertions of personal autonomy when they otherwise had none.

Comparing abortion to slavery brushes aside that history — especially the history of rape, which some Republicans (including front-runner Ted Cruz) don't should be a reason for legal abortion. It also trivializes women's need for autonomy, especially black women's.

The anti-racism arguments in bills like PRENDA are particularly baffling, pro-choice advocates say. They talk about how many black or Hispanic babies have been murdered by abortion. They talk about "black genocide." But these arguments suggest that women of color who choose to have an abortion, for whatever reason, are being racist against their own fetus.

Abortion opponents "think it’s clever to compare abortion to slavery or the Holocaust ... yet they fail to look at what actually happened during those times," said reproductive justice advocate Renee Bracey Sherman, who is a biracial black woman. "That’s offensive to all of our ancestors, and erases the very real experiences of black women today."