Dr. Tiller sought advice from security experts, who gave him tips, such as to drive in the right lane to limit the angles available to potential shooters, and he started giving his employees “combat pay” bonuses to keep them from quitting. He eventually bought an armored S.U.V. to drive to and from the clinic, and for two and a half years federal marshals were by his side for much of each day. He also varied his route between the clinic and his home, where he lived with his wife and four children.

He took these precautions because the threats that his clinic received were not all idle. The facility was pipe-bombed in 1986, causing $100,000 in damage. And in 1993, Dr. Tiller was shot in both arms by an anti-abortion extremist named Rachelle Shannon. (Ms. Shannon, who also committed a series of clinic bombings and acid attacks, was released from prison last year.) Neither the bomb nor the shooting deterred him; he was back at work the day after the shooting. Nor was he cowed when thousands of protesters, some of them aggressive, descended on his clinic and tried to block access to it during the so-called Summer of Mercy in 1991.

Dr. Tiller also stayed strong while Bill O’Reilly, the erstwhile Fox News personality, went on a nationally broadcast campaign against him in the years before his murder. Mr. O’Reilly compared Dr. Tiller to Hitler, said he was “executing babies” and noted in 2006: “If I could get my hands on Tiller … Can’t be vigilantes. Can’t do that. It’s just a figure of speech.”

A few years later, of course, someone did get his hands on the doctor .

(After Dr. Tiller’s death, Mr. O’Reilly said that “Americans should condemn” his murder and that “anarchy and vigilantism will destroy a society.”)

Today, abortion providers face an even more volatile political backdrop than Dr. Tiller did during his lifetime. In the years after Dr. Tiller’s murder, state legislatures passed hundreds of anti-abortion regulations intended to shut down abortion clinics and make it harder for women to access the procedure. Then came Donald Trump, who became president thanks in large part to the support of evangelical voters counting on him to deliver anti-abortion Supreme Court justices and other judges — a promise that he has fulfilled, leading anti-abortion lawmakers in states around the country to pass a rash of near-total abortion bans this year.

Since the 2016 election, abortion providers have reported a spike in incidents of vandalism, trespassing, harassment and picketing. According to the National Abortion Federation, which tracks such data , in 2018 abortion providers were subject to at least 1,135 trespassing incidents in the United States and Canada — up from 823 incidents in 2017 and 264 in 2013, when there were more abortion clinics in America than there are today . And last year they experienced nearly 122,600 disruptive events, including internet harassment, bomb threats and picketing. In 2017, that number was fewer than 97,000, and in 2013 it was fewer than 6,500.