Twitter has been fighting hate speech for years. Now, it faces what may be the world’s most powerful troll: the president of the United States.

President-elect Donald Trump used Twitter for the past 18 months as a megaphone for his views and rants. He’s attacked the media, insulted women, praised himself, and lied outright. One of Trump’s recent claims— “I won the popular vote if you deduct the millions of people who voted illegally”—has already been rebuffed by Republicans, Democrats, fact-checkers, and election officials. Trump has produced no evidence to support it.

While Trump’s deceptive tweets may not violate Twitter’s rules against harassment, threats and “hateful conduct,” Twitter is still keeping an eye on his account for more egregious offenses. This week, the company told Slate it would consider banning key government officials, even the president, if its rules against hate speech or other language were violated. “The Twitter Rules prohibit violent threats, harassment, hateful conduct, and multiple account abuse, and we will take action on accounts violating those policies,” a spokesperson wrote. Twitter confirmed with Quartz that everyone, including government officials, were subject to the policy: “The Twitter Rules apply to all accounts,” a spokesman wrote.

Trump may not have crossed that line yet, but he hasn’t exactly refrained from making incendiary claims. Most recently, he claimed that Abdul Razak Ali Artan, who allegedly carried out an attack injuring 11 students at Ohio State University, “should not have been in our country.” Artan was a legal permanent US resident, whose family had fled Somalia for Pakistan in 2007. He arrived in the States in 2014.

Even Republicans have grown wary of Trump’s unfiltered, potentially inflammatory tweets. Newt Gingrich, an adviser and vice-chairman of Trump’s transition team, said the president-elect’s claim about illegal voters was out of line. “The president of the United States can’t randomly tweet without having somebody check it out,” Gingrich told USA Today. “It makes you wonder about whatever else he’s doing. It undermines much more than a single tweet.”

Twitter has been firm about—if not exactly speedy on—removing fairly prominent accounts in the past. Milo Yiannopoulos, a right-wing troll and technology editor for Breitbart.com, was permanently banned from the platform after allegedly inciting social media attacks on actress Leslie Jones. The social network also banned a prominent governor in Venezuela’s ruling Socialist party for apparently tweeting private information.