“I fought Planned Parenthood and we stopped the sale of baby body parts,” said Rep. Marsha Blackburn (R-TN) in a recent ad announcing her run for Senate.

Under ordinary circumstances, that line might be politically controversial — Blackburn was referring to her work with a House panel on fetal tissue research that one Democratic member of Congress called “an inappropriate and wasteful misuse of federal resources.”

But Blackburn’s statement became part of a different kind of controversy on Monday after it got her campaign ad blocked by Twitter.

Though Blackburn’s claim that she “stopped the sale of baby body parts” is certainly inflammatory, Twitter’s decision to block the ad is surprising since the service has often been slow to ban abusive and threatening users. Twitter has explained that it has different, stricter rules for advertisers than for ordinary users. But critics of the move argue that by banning the ad, Twitter has simply given its message more power.

Blackburn’s claim about “baby body parts” goes back to a series of hidden-camera videos

To understand Blackburn’s claim, you have to go back to 2015, when an anti-abortion group called the Center for Medical Progress released hidden-camera videos of Planned Parenthood employees discussing fetal tissue research. Such research is legal, and researchers say it is helpful in developing treatments for diseases like HIV and Parkinson’s disease. However, it is illegal to sell fetal tissue for a profit.

The Center for Medical Progress claimed the videos showed “Planned Parenthood’s sale of baby body parts.” In fact, although the videos did include matter-of-fact discussions of fetal tissue that were off-putting to some viewers, they did not provide any proof that Planned Parenthood sold fetal tissue for a profit. Planned Parenthood officials make clear in the videos that fetal tissue, which abortion patients can opt to donate for medical research, is not a source of profit for the organization, and that clinics only accept reimbursement for the costs of tissue collection, which is legal.

The videos were edited to make Planned Parenthood look worse. The Center for Medical Progress released longer videos it described as “full footage” as well, but Planned Parenthood has said there is evidence these videos were edited as well.

Activists at the Center for Medical Progress were able to meet with Planned Parenthood employees by posing as representatives of a fake tissue procurement company. According to Planned Parenthood, the fake company went so far as to set up exhibits at Planned Parenthood conferences.

David Daleiden, the founder of the Center for Medical Progress, has faced a number of criminal charges as a result of the videos. In Texas, he was charged with tampering with a government record (for making a fake driver’s license to deceive Planned Parenthood employees) and attempting to buy human tissue. Those charges were later dismissed. In California, Daleiden and an associate were charged with 14 felony counts of recording people without their permission and one count of conspiracy to invade privacy, according to the Los Angeles Times. A judge dismissed some of those charges, but in July, prosecutors filed new ones.

Though the videos were recorded under false pretenses, they inspired a real reaction. At least a dozen states launched investigations into Planned Parenthood, but none of them found any wrongdoing, according to the Los Angeles Times. Several states moved to strip funding from the organization. And the House formed a panel, chaired by Blackburn, to look into the handling of fetal tissue nationwide.

In a report issued in January, Blackburn and the other Republicans on the panel concluded that a number of organizations “may have been profiting from the sale of baby body parts” and noted that they had asked state and federal authorities to investigate further. They also argued that fetal tissue was not important for medical research. The report recommended banning federal funding for research on tissue from aborted fetuses, stripping federal funding from Planned Parenthood, and enacting a 20-week abortion ban.

Democrats on the panel, meanwhile, charged that it was essentially a sham. “By all appearances,” Emily Crockett wrote at Vox last year, based on the Democrats’ report and her own reporting, “Republicans on the select committee have abused their subpoena power to intimidate doctors and medical researchers, flouted House rules and traditions, and used shoddy evidence to promote a predetermined, partisan conclusion instead of making any genuine efforts at fact-finding.” In Science, Meredith Waldman challenged the panel’s conclusion that fetal tissue had little medical use, noting that medications for rheumatoid arthritis, cystic fibrosis, and hemophilia have been created using fetal tissue.

And some were concerned that the panel’s tactics — especially requesting the names of individual fetal tissue researchers and abortion clinic employees — could endanger people. “Not only do I believe that this panel is an inappropriate and wasteful misuse of federal resources, but I am gravely concerned that it also puts researchers, providers and patients across this country at risk,” said Sen. Jeanne Shaheen (D-NH), according to the Tennessean.

Twitter had a reason for its decision — but that doesn’t mean it was a good one

Of course, none of the above really explains why Twitter would ban Blackburn’s ad. The service decided to block the ad, according to the Associated Press, because the comment about “baby body parts” was “deemed an inflammatory statement that is likely to evoke a strong negative reaction.” That’s hard to argue with. But Twitter has been repeatedly criticized for failing to ban abusive and threatening users. Last year, Leslie Jones received an onslaught of racist and abusive tweets, and while the service has made changes, any regular user knows that racist, sexist, and anti-Semitic insults are disturbingly common.

So why block Blackburn’s ad? The explanation, according to Twitter, is that the service has different rules for paid ads than for ordinary user tweets. “Twitter provides a platform for its users to share and receive a wide range of ideas and content, and we greatly value and defend our users' ability to express themselves,” the company’s advertising policy reads. But because of Twitter’s targeting options, “advertisers on Twitter have the power to reach an audience beyond the users who choose to follow their account." With greater power comes greater responsibility — Twitter requires advertisers to adhere to stricter standards than users, and reserves the right to ban ads that include “inflammatory or provocative content which is likely to evoke a strong negative reaction.”

A spokesperson for Twitter confirmed to Vox that while the promoted tweets containing Blackburn’s ad were banned, Blackburn was still allowed to tweet the ad from her ordinary user account.

Twitter’s move here appears consistent with its advertising policy. But critics argue it will have the opposite of the intended effect, directing more attention to Blackburn’s message. Sahil Kapur, a political reporter for Bloomberg News, called the move essentially “an in-kind contribution to Marsha Blackburn’s campaign.” Twitter’s decision certainly resulted in widespread media coverage of the ad — at the Washington Post, Politico, and elsewhere — which it might not have otherwise received.

The ban also gave Blackburn the chance to fire back at “Silicon Valley elites,” whom she accused in a fundraising email of “trying to impose their values on us.” “I’m being censored for telling the truth,” she wrote in the email, according to Politico.

Twitter has reportedly banned ads in the past — the anti-abortion group Live Action said earlier this year that the service had blocked their ads because Live Action’s website and Twitter feed included “sensitive content” like images and videos of abortion procedures. As Tracy Jan wrote at the Washington Post at the time, “companies have the right to set their own guidelines, even if it means blocking ads promoting controversial political or social issues.”

But Twitter may find that it needs more specific guidelines where political ads are concerned. After all, nearly all ads for political candidates will “evoke a strong negative reaction” from someone.