Mikey Dickerson, the executive director of the New Data Project and a former Google engineer who was the chief of the United States Digital Service in the Obama administration, had learned about the Michigan survey in 2008, but he didn’t come up with his VoteWithMe app until 2017. The app outs anyone who didn’t vote in previous elections. (Alyssa Milano and Alexandria Ocasio-Cortez, looks like you both sat out the 2014 midterms.)

It identifies friends in competitive districts (with a fire emoji) and provides handy text-message reminder templates (“You gonna vote?”). “We looked around and didn’t think it had ever been done before — putting voter-file data directly in front of you as the end user,” Mr. Dickerson said.

Many in the technology industry initially scoffed at his idea. (“VoteWithMe is a creepy new app that checks your contacts’ voting history,” one article declared.) But several hundred thousand users downloaded VoteWithMe before the midterms, and Mr. Dickerson, who plans to step back and hand the technology over to like-minded groups, said lots of similar apps are springing up ahead of 2020.

“I am worried about the foundations of the systems of government surviving and an administration that puts kids in cages and separates families,” he said. “I’m not going to feel bad that some of my friends, unaffected by any of this, have a mild amount of discomfort because some jerk — probably me — can see whether or not they voted.”

The weight of peer pressure has a particular pull on millennials. They represent more than 30 percent of eligible voters, about on a par with baby boomers, but have the lowest voter turnout of any age group. Only 49 percent of voters ages 18 to 35 voted in the 2016 presidential election, according to the Pew Research Center. Democratic pollsters predict that increasing turnout among millennials by 10 percentage points would all but guarantee they defeat Mr. Trump in 2020.

In an era when privacy feels like a nostalgic notion and our political leanings can be more or less gleaned from where we live, how we dress and what we watch, is there even such a thing as the sanctity of the voting booth? After several celebrities learned the hard way that most states don’t permit photos inside the voting booth, last year California legalized “ballot selfies” (thank you, Kim Kardashian) . The day of the midterm elections, 1,000 people a minute were posting Instagram stories with “I Voted” stickers, according to the company.

Developers said these turnout apps aren’t intended to shame anyone. As Debra Cleaver, the chief executive of the San Francisco-based Vote.org, a nonprofit group that works to increase voter participation, put it: “We call it social pressure or social validation. ‘Vote shaming’ sounds like it was coined by a reporter because it makes you want to click.” (Fair.)