On January 18th, the Twitter account for a new political organization posted its first tweet: a link to swingleft.org—a neatly designed Web site where you can plug in your Zip Code to find the nearest U.S. House district whose seat was, in the most recent election, decided by a small margin—along with the message “Let’s get to work.” The Swing Left campaign, which aims to win the House for Democrats in 2018, quickly went viral. The comedian Sarah Silverman tweeted “Start thinking mid term elections now - this makes it CRAZY easy,” with a link to the site. As the roughly three million people who came out for the Women’s March on Saturday made colorfully evident, an enormous, amorphous bundle of progressive energy in the country is searching for an outlet or three. By January 22nd, a hundred thousand people had signed up to receive Swing Left updates. That number has since more than doubled. In addition, ten thousand people have filled out a form on the site to offer their skills in a volunteer capacity. The Web site has been shared on Facebook nearly three hundred thousand times.

Swing Left is the brainchild of Ethan Todras-Whitehill, a “writer, GMAT teacher, dad, and political nerd” who lives in Amherst, Massachusetts. “Like a lot of people, after the election, I was flabbergasted and devastated,” he told me this week, over the phone. “But I work through stages of grief pretty quickly.” The morning after Donald Trump’s victory, as he sat in a local coffee shop, he reached acceptance: the Trump Presidency was real. “The way forward is to do something in 2018,” he said. “And it’s a very bad map for Democrats in the Senate in 2018, but House elections tend to swing against the incumbent, particularly when one party controls all three branches. All of this meant that 2018 would be a prime opportunity for Democrats to take back the House.”

Todras-Whitehill lives in a solidly blue area, with no immediate opportunities to flip or meaningfully defend a congressional district. “No Republican ran for office around here—they didn’t even bother—and a lot of progressives live in districts like that,” he said. So he went home and perused CNN’s Web site to find the closest district where the margin of victory was close. It was New York’s 19th Congressional District, where the Republican John Faso defeated Zephyr Teachout, a Democrat, by fewer than thirty thousand votes in November. “I was getting ready to post on Facebook, to say that I would commit my time and energy to flipping N.Y. 19 in 2018,” he said. “But then I wondered, Why did I just have to do that? Why doesn’t a tool for finding your nearest swing district already exist?”

He called his best friend, Josh Krafchin, a “developer, dad, and entrepreneur” who now lives in the Bay Area and whom Todras-Whitehill has known since high school. “I was like, ‘Josh, you have to build me this tool, or we have to find someone to build it.’ ” Krafchin took to the idea immediately, and brought his wife, Miriam Stone, a brand strategist, on board. The three of them reached out to developers, designers, and friends of friends, working toward having something ready by Inauguration Day. It made them feel like they were doing something other than watching from the sidelines. “We felt empowered, and we want to deliver this feeling of empowerment to progressives—to Bernie supporters, Hillary supporters, anyone who’s scared about the direction this country is headed,” Todras-Whitehill said.

Swing Left identifies swing districts through a simple calculation: the congressional districts whose seats were decided within a margin of fifteen percentage points. I entered my Brooklyn Zip Code and Swing Left gave me New York’s Third Congressional District, centered in northern Nassau County, where the Democrat Tom Suozzi, a first-time congressman, won his seat in 2016 by a little more than seventeen thousand votes. In other areas of the country, the districts to which Swing Left would direct you are implausibly far from home: someone in Seattle, say, would get a district in rural Nevada. The interface is, for now, simple: Swing Left shows the district boundaries and the name of the current representative and then prompts the user to sign up for a mailing list to receive more information in the future.

Looking at the organization’s map, I noticed that the district in Houston where I grew up—Texas’ Seventh—was marked as a swing district. John Culberson, the Republican congressman to whom I would write letters about NASA when I was in elementary school and he was in the State House, defended his seat in 2016 by more than thirty-one thousand votes. Culberson chairs the House Appropriations Subcommittee on Commerce, Justice, Science, and Related Agencies, and for the past two years he has been laying the groundwork for the removal of federal funding from sanctuary cities. He’s best known nationally, perhaps, for exclaiming, “Like 9/11, let’s roll!” in reference to the Republican commitment to delay Obamacare funding, in 2013. Houston has gotten increasingly blue in recent years: Harris County, which contains the Seventh District, voted for Hillary Clinton by a margin of more than twelve points. But incumbents in Congress have serious advantages: in 2012, when congressional approval rates hovered around fifteen per cent, ninety per cent of House representatives were reëlected. Culberson, an eight-term incumbent in a district that was recently redrawn to his favor, in a seat that has been held by a Republican since George H. W. Bush flipped it, in 1966, is deeply entrenched.

“We do know that fifteen per cent is a blunt-force instrument,” Todras-Whitehill said. “I’m enough of a political nerd to understand that there are better ways of gauging a swing district. I understand that our metric will catch districts that are potentially unwinnable. But I wanted the initial criteria to be accessible, I didn’t want to put my finger on the scale, and I wanted to cast a broad net.” The possibility of flipping the House needed to seem tangible. He knows that the meaning of a margin depends on a district’s context, and said that he hoped to encourage a public dialogue about candidates in the districts that Swing Left had identified, so that people with specific knowledge of these battleground seats could shape the site’s instructions. “We will remove some districts and add some districts. We’ll transfer people who signed up, in a way that’s appropriate, and we’ll just be totally transparent about all of it.”

The importance of transparency was publicly impressed upon Swing Left’s founders within a few days of the Web site’s launch: they didn’t initially identify themselves anywhere on the site, and that—along with the staggering, immediate response—prompted skepticism in some corners of the Internet. A community blogger on Daily Kos wrote a post headlined “Swing Left Is Not to Be Trusted as a Progressive Resource.” The blogger noted, gratuitously, that Krafchin’s former business partner had a Russian name, and conjectured that Swing Left might be trying to run “unverified” candidates against Democrats, to split the progressive vote. I asked Todras-Whitehill if he was an agent of the Russian state. “Not to my knowledge,” he said, laughing. “But that’s life on the Internet these days. And it’s good that our community had questions. We heard them, and we’re committed to transparency.” This week, the founders put their names on the site’s About page, and Krafchin posted an open question on Quora, asking what Swing Left should add to its map.