Democrats have already pulled off one unthinkable ploy this year in Kansas, dropping their own candidate and leaving an independent (who sounds a lot like a Democrat) who is currently holding a solid lead against the GOP incumbent.

Now things are getting interesting in South Dakota, where Sen. Tim Johnson (D) is not seeking re-election and which until now had been written off as a Republican gain in November. Big money from an unexpected source is starting to spill into the race to give Democratic candidate Rick Weiland a boost, and polling shows that Republican candidate Mike Rounds hasn’t been able to build a foolproof lead with an independent candidate muddying the campaign.

MayDay PAC, the pro-campaign finance reform super PAC that has set out to eliminate super PACs, announced on Monday that it would spend $1 million to support Weiland in his bid against Rounds — while the independent candidate, former GOP Sen. Larry Pressler, further complicates the race.

Some recent polling prompted MayDay PAC to get into the South Dakota campaign, its founder, Harvard University professor Lawrence Lessig, told TPM in an email. A Survey USA poll released on Tuesday found Rounds, a former governor who has been dogged by a scandal about alleged corruption involving immigration visas issued during his administration, at 35 percent, Pressler at 32 percent, and Weiland at 28 percent.

The TPM PollTracker average puts Rounds at 35 percent, Pressler at 29.3 percent and Weiland at 28 percent. So Weiland is fighting an uphill battle — but with a third candidate messing with the usual party dynamics and MayDay PAC and its allies sending in support, Lessig thinks he can pull it off.

Weiland has a “clear path to victory, if the campaign is executed properly with the right resources,” Lessig said in an email.

MayDay PAC’s $1 million is a serious start. According to a TPM review of Federal Election Commission data, outside groups have spent about $350,000 in South Dakota since Sept. 1 — and all of that came from Every Voice Action PAC, another campaign-finance-centric group that is allied with MayDay and backing Weiland. National Democrats have been almost entirely absent, which led to a public spat between former Senate Majority Leader Tom Daschle (D-SD) and current Senate Majority Leader Harry Reid (D-NV) over the lack of support for Weiland.

The opening seems to stem largely from Rounds’ weakness as a candidate. According to Monday’s SurveyUSA poll, he is attracting only 55 percent of GOP voters, and more than half of voters said he needed to explain more about the immigration-visa scandal. It has election prognosticators like Five Thirty Eight’s Nate Silver openly wondering if the seat might actually be in play.

More twists could be coming — after all, a candidate withdrawal has already shaken up another Senate race in Kansas. Polls show that Pressler, a former Republican who endorsed Obama in 2008 and 2012 and hasn’t said whom he would caucus with, would have a huge edge on Rounds if Weiland dropped out. Weiland, on the other hand, would draw even with Rounds without Pressler, according to the new SurveyUSA poll.

Lessig is keeping an open mind. He told Vox that MayDay’s campaign would be pro-Weiland and anti-Rounds, but wouldn’t touch Pressler.

And the payoff for Lessig and MayDay wouldn’t just be a Weiland win. If they swoop in and save a Senate seat for Democrats, he thinks it could go a long way toward legitimizing what is otherwise seen as a quixotic quest to reform campaign finance laws through the electoral process.

“If we can move him to victory (or even close) on our issue,” Lessing told TPM, “it demonstrates what we’re working to show: that voters care about this issue.”