The backroom conversations politicians have with their base always seem to come back and bite them in the ass.

You still remember Mitt Romney’s infamous "47 percent" remarks, when he told a room full of well-heeled donors during the 2012 campaign that 47 percent of the electorate are “dependent upon government,” “believe that they are victims,” and would vote for President Obama no matter what. And surely you haven’t forgotten Hillary Clinton’s Goldman Sachs speeches yet, in which she expressed her desire for “open trade and open borders” before an audience of global financiers, precisely the policies and people President Trump’s supporters most violently opposed.

Now, through the terrifying wonders of social media, President Trump is doing much the same thing with his targeted digital ads—only on a much larger scale.

Buzzfeed News highlighted one example on Wednesday when it reported that President Trump’s campaign has been targeting supporters with Facebook ads, vowing to build the wall on the southern border, despite “rumors” to the contrary. (In fact, those "rumors" come from a statement put out by Sen. Chuck Schumer and Rep. Nancy Pelosi, reporting they had reached a border security deal with Trump that did not include a wall.) These ads don’t appear as regular News Feed posts on Trump’s page. Instead, they’re only visible to the Facebook users his campaign targeted.

Pandering to the base is a tradition as old as politics itself. But in the social media age, it’s easier than ever for politicians to take those tailored messages—the kind they might not like to share with the whole world—and disseminate them only to the people who are most likely to agree. And targeting allows campaigns to silo thousands of possible audiences with just a click, making it harder than ever to hold politicians accountable for all of it.

Some have taken to calling this type of ad a “dark post,” an overly nefarious name for what is, in actuality, just the way digital ads operate today. Technically speaking, Trump's ad buy works the same as one for the pair of Zappos shoes that somehow follows you around the internet. You’re seeing those shoes because Facebook thinks you're in the market for shoes. It's no different either from the ads Hillary Clinton’s campaign served up, say, reminding Iowans to vote in the primary. They wouldn't have shown up for Democrats in Texas.

“The benefit of digital advertisements is being able to give people information that’s relevant to them,” says Andrew Bleeker, president of Bully Pulpit Interactive, who ran Clinton’s digital advertising during the 2016 election. “There’s no ill intent. That’s just how it works.”

A few factors still make Trump's border security ad unnerving, though. Trump isn’t running a shoe store; still less than a year into his term, he's already running a reelection campaign. And when the president sends one subset of the population a message that the rest of the population can’t see—especially one that's at odds with reality—it feels like a fundamental failure of government transparency.

Sure, politicians have always pandered to private parties, sent different mailers to different voters, and aired different television ads in different parts of the country. But on social media, infinite combinations of possible audiences are only ever a click away, and campaigns can develop thousands of highly specific messages for all of them. That makes tracking the messages within those ads nearly impossible.

“To call it ‘dark posts’ is misleading, but there is a legitimate question about accountability and transparency in political advertising,” says Nicco Mele, author of the book, The End of Big: How the Internet Makes David the New Goliath, and director of the Shorenstein Center on Media, Politics, and Public Policy at Harvard.