Minutes after House Republicans rammed through their Obamacare repeal bill on May 4, a rallying cry went out on Twitter: Want revenge on the lawmakers who voted yes? Click here to give $5 to defeat them in next year’s midterms.

Stalwart lefty groups like Daily Kos and anti-Trump newcomers such as Swing Left mobilized their angry followers by sending them to a simple web page where, with the press of a button, they could donate to the Democratic challengers of Republicans in competitive districts. In just 24 hours, Daily Kos and Swing Left announced they’d collected about 37,200 individual donations totaling a whopping $1.6 million.

This impressive blitz was made possible by an outfit called ActBlue, which has emerged as the fundraising tool of choice for the swelling anti-Trump resistance. At ActBlue’s headquarters just outside Boston, a massive LCD monitor that displays website traffic and incoming contributions went haywire immediately after the health care vote. ActBlue was processing about 275 donations per minute; the real-time dollar ticker on its website struggled to keep pace. All told, ActBlue helped raise more than $4.2 million in small-dollar increments for 1,200 different campaigns and outfits on the day of the vote.

Under normal circumstances, a $4.2 million day in May of a nonelection year would be inconceivable. But these aren’t normal times, and ActBlue has arguably become the best weapon in the Democratic arsenal as progressives clamor to fight back. Indivisible has raised $2.2 million from over 30,000 donations, the vast majority of it through ActBlue. Jon Ossoff, the upstart Georgia Democrat running to fill the seat of Health and Human Services Secretary Tom Price, has used ActBlue to help raise $23 million (and counting), enabling him to mount a serious campaign for a district that’s been in Republican hands for almost 40 years. “There is zero chance that we would be able to raise this much money on any other platform,” says Greg Berlin, a consultant for Ossoff’s campaign.

On paper, it’s a straightforward concept: ActBlue works with candidates at the local, state, and national levels—from school board races to presidential campaigns—to squeeze every dollar out of their email fundraising pleas or the ubiquitous “Donate” button on their websites. Engineers streamline the process of giving to a campaign or cause. They toy with typefaces, reduce load times, and adapt the product to all devices and operating systems. Like an Olympic sprinter in training, ActBlue obsesses over shaving off every millisecond. “If you want people to take action,” says Erin Hill, ActBlue’s executive director, “you need to make it incredibly easy for them to do it.”

ActBlue is a nonprofit that charges nothing for its services. (It takes 4 percent of every donation to cover credit card processing fees.) Operating costs are paid with tips left by donors and the occasional fundraising campaign. Anyone who uses it—ActBlue only serves candidates and causes on the left—gets access to the same technology. “If you’re running for city council in Peoria, Illinois,” says Nate Thames, who runs ActBlue’s team of engineers and technicians, “you get the exact same set of tools that Bernie Sanders used to run for president.”

Founded in 2004, ActBlue was born out of an earlier moment of crisis for Democrats: the reelection of George W. Bush and the Iraq War. An MIT computer scientist, Matt DeBergalis, and a Harvard-trained physicist, Ben Rahn, envisioned a way for candidates to raise money outside the traditional rubber-chicken dinners and big-dollar galas. And they wanted to remedy an all-too-common dilemma in US politics: individual campaigns building their own tools and programs only to see any innovations mothballed after the election cycle is over.

ActBlue, by contrast, is a permanent part of Democratic politics. (Strangely enough, Republicans have failed to replicate it.) The data generated by its users goes right back into crafting new ways to raise cash. For instance, an individual congressional campaign couldn’t justify the expense of trying to integrate Apple Pay into its online fundraising. But ActBlue sank five months of engineering time into doing just that and has raised almost $6 million using Apple Pay since October. “We do things that for any single campaign would make no sense,” says Thames. “When you put it across the entire left, it makes a lot of sense.”

The most high-profile ActBlue user is undoubtedly Sen. Bernie Sanders (I-Vt.). Hill says Sanders’ success still resonates because it showed it was possible to mount a viable bid for the White House funded entirely by small donors. Sanders “was outraising Clinton most months in $27 increments,” says Hill, referring to the average contribution given to the Sanders’ campaign. “That had never been done before.” (Hillary Clinton’s campaign built its own fundraising tools but used ActBlue near the end of the 2016 race.)

A week before the May 4 health care vote, I visited ActBlue’s peach-colored headquarters in Somerville, Massachusetts. It has the feel of a startup—Ping-Pong table, ample snack options—crossed with a campaign field office. Conference rooms are named after fictional US presidents (Dr. Strangelove‘s Merkin Muffley: “Gentlemen, you can’t fight in here! This is the War Room”). On the first floor, a net hangs overhead. It’s a reminder of a key moment: Back in March 2016, after 12 years in business, ActBlue raised its billionth dollar, and the staff celebrated with a convention-style balloon drop. So energized is the grassroots left that it took just 10 months for ActBlue to raise another half-billion dollars.

The company employs 50 people in Somerville and around the country. To keep up with the growing demand, Hill told me she plans to add 30 more employees by summertime. She also expects ActBlue to hit the $2 billion mark later this year. When I asked her and Thames how they intended to celebrate, they exchanged glances as if, in all the craziness of late, they hadn’t thought about what to do when that rapidly approaching milestone arrived.

“Maybe we’ll need confetti cannons?” Hill offered.

Thames paused to consider it. “That would be such a mess to clean up.”