We all know super PACs have permanently changed the face of American politics. But in South Carolina, right now, we’re getting a preview of the unprecedented ways they can alter a campaign.

South Carolina, as you may know, is kind of a big deal. It has a large population—4.8 million people—and a bad reputation because it’s a place where candidates love to break out their dirtiest tricks.

Or, in the case of Ted Cruz’s super PACs, their most experimental. The cluster of well-funded super PACs boosting Cruz’s candidacy is trying out a new tactic in the Palmetto State, indicating the extent to which super PACs are encroaching on traditional campaign turf.

And it has Cruz’s rivals scared.

Said super PAC, called Keep the Promise—which is actually sub-divided into several different PACs, each funded by a different billionaire family—has blithely tossed the traditional super PAC playbook to the winds. In fact, they’ve taken on typical campaign operations: gathering voter data, targeting likely Cruz supporters, and knocking on thousands of doors to get out the vote.

The super PAC has had upwards of 250 people canvassing the state, targeting the homes of persuadable Republican voters. Thus far, they estimate they’ve knocked on more than 93,000 doors. And by Election Day, they’re shooting to have knocked on 100,000. In any given week, they say, 100 to 150 individual people spend eight-hour days doing the door-knocking. And most of them get paid.

Traditionally—to the extent that we have age-old super PAC traditions—super PACs pay for expensive TV and radio ads. Federal election law forbids them from coordinating with the campaigns they try to boost, so shelling out big bucks to produce and air TV ads has been their natural role. And that’s what most of them have done in this cycle—from a pro-Rubio super PAC running birther-esque ads ominously intimating that Cruz is darkly influenced by his Canadian roots, to the pro-Bush Right to Rise PAC, which got President George W. Bush to star in a spot boosting Jeb.

That’s the norm. Campaigns run events, corral volunteers, and staff regional offices; super PACs slap up ads.

South Carolina politicos describe it as an effective, relentless operation. And it has some of Cruz’s opponents feeling a little jittery.

“I’ll be very shocked, honestly, if Ted Cruz doesn’t win the primary,” said an operative for a rival campaign, citing Keep the Promise’s blanketing of the Upstate.

Trump has led by double digits in all the recent Palmetto State polls. But some are skeptical that his lead is really that commanding. And they point to the different ground games—particularly, to that of Keep the Promise—as evidence for their doubt.

Keep the Promise staff explained that the group has been door-knocking across the state, in a few targeted regions and counties, since last November. In early January, those door-knockers started focusing on persuasion: identifying likely Republican primary voters who favor an Evangelical Christian candidate, knocking on their doors, and having conversations aimed at persuading them to back Cruz.

“What we are doing right now is what I dreamed about doing as Scott Walker’s state director,” said Dan Tripp, who formerly helmed Walker’s South Carolina operation and now runs the show there for Keep the Promise.

“It’s hard, dirty work,” Tripp said. “If we’re asking somebody to go out and knock on doors for eight hours, that’s a lot of gas, that’s a lot of time and it’s hard work. So we’ve built a budget around paying our canvassers.”

The ease with which super PACs can raise money—no contribution limits, no spending limits—means groups like Keep the Promise have resources that campaigns may not. In the case of Keep the Promise, that’s thanks to a handful of billionaire families who decided Cruz was their guy. One Keep the Promise backer, Robert Mercer, is a hedge fund billionaire who keeps a very low profile and has drawn criticism for tax-dodging. Keep the Promise supporters also include Dan and Farris Wilks, brothers from Central Texas who made their first billion thanks to the fracking boom and have invested their wealth in supporting social conservative groups and candidates.

Thanks to their largesse, Keep the Promise has been able to pay a mobile army of door-knockers and voter-persuaders.

“There is nothing fun about doing a get-out-the-vote push,” said Katon Dawson, formerly a Rick Perry operative in South Carolina, noting that he was also impressed by the super PAC’s efforts. “It’s the hardest work you can do in any campaign. It’s hard, it’s expensive.”

Now, the Cruz campaign itself is also doing door-knocking and canvassing. But Tripp said he isn’t worried about redundancy; while voters often get irritated by millions of robo-calls about a candidate, they’re less likely to snap at someone at the face of a sympathetic human being slogging through their neighborhood in January. And the PAC may have an even farther reach than the campaign proper

Matt Moore, the chairman of the South Carolina Republican Party, said the PAC may have an even farther reach than the campaign proper since Keep the Promise is the first and only long-term, large-scale super PAC canvassing operation in the state.

And he knows firsthand just how effective it is.

“His team even appeared at my parents-in-laws home in suburban South Carolina,” he said. “To my knowledge, they’re the only campaign that has been to their home.”

He added that Keep the Promise is the only super PAC he knows of that has any sort of ground game in the state. Paul Lindsay, a representative for Right to Rise, said his PAC also has a professional canvassing operation that recently moved down from New Hampshire to South Carolina. But it hasn’t made nearly the splash with locals as Keep the Promise.

And the Keep the Promise team is pretty confident that their ground game tops that of their closest rival, Donald Trump. The mogul’s team has sent RVs of volunteers around the Upstate door-knocking. But Tripp said those efforts are more about marketing than anything else.

“We’re not gonna punch down when it comes to the ground game,” he said of the Trump efforts. “Honestly, I don’t think they’ve been much competition at all.”