Money is pouring into South Dakota’s U.S. Senate race as people and groups across the country sense a rare chance to support a potentially competitive challenger against a Republican incumbent.

Both candidates have raised a majority of their campaign contributions from outside the state and have spent a combined $6.1 million, according to data compiled by the nonpartisan, nonprofit money-in-politics website OpenSecrets. Additional spending by federally registered political action committees to influence the race has topped $3 million, pushing the total cost of the race near $10 million and counting.

In a race with no Democratic candidate, about 40% of the spending has been in support of independent Brian Bengs or against Republican Sen. Mike Rounds.

Independent US Senate candidate in South Dakota backed by Forward Party

Bengs trailed Rounds by 8 points in an Emerson College/KELO-TV poll released last week, with 8% of respondents undecided. Another poll released last week, from Mason-Dixon Polling and Strategy and South Dakota News Watch, showed Rounds with a larger lead of 19 points, 56% to 37%.

Nationally, Bengs is one of four independent candidates for U.S. Senate in the Nov. 3 election who are the main opponents of Republican senators. All of them are in states where Trump has won and Democrats struggle in statewide elections. The other states are Nebraska, Montana and Idaho.

In South Dakota, Democrats are outnumbered more than 2-to-1 by Republicans. South Dakota Democrats have also been surpassed in recent years by the number of voters who register as independent or leave their political affiliation blank on the voter registration form.

Federal committees and nonprofits

Much of the advertising in the campaign’s final weeks has been funded by groups immune from state-level contribution limits, with varying levels of disclosure about their donors. That includes $1.7 million spent to promote Bengs or oppose Rounds by the federally registered political action committees Retire Career Politicians and Freedom from Politics.

Retire Career Politicians has received large recent contributions including $890,000 from the Western Futures Fund, a nonprofit that says it promotes “responsible leadership in the Western United States” and is not required to disclose its donors; $500,000 from the Way Back PAC, which has received much of its own funding from the Western Futures Fund; $200,000 from Scott Shenker, an internationally known computer scientist from California; and $100,000 from Brennan Bilberry, a political and communications strategist from Massachusetts.

Freedom from Politics Inc. has received all of its $1.2 million from Stephen Pajcic, a Florida lawyer, Democratic politician and philanthropist. The committee’s stated mission is to “support independent-minded leaders who answer to the people they represent, not to a political party.”

On the other side, the South Dakota Conservative PAC has spent nearly $1 million while Defend American Jobs has spent about $400,000 to support Rounds or oppose Bengs.

South Dakota Conservative PAC received $200,000 from the late South Dakota billionaire-philanthropist T. Denny Sanford before he died, among other large individual contributions. Recently, the committee received an infusion of $521,000 from South Dakota First Action Inc., an entity that divulges little about itself publicly but describes itself as a nonprofit dedicated to “promoting policies that promote free enterprise, strong national security, and individual liberty.”

Defend American Jobs’ contributions include a recent $70 million surge of funding from Fairshake, a Minnesota-based political action committee that says it “supports candidates committed to securing the United States as the home to innovators building the next generation of the internet.” Contributors to Fairshake include the California-based tech company Ripple Labs Inc. and Marc Andreesen, a leader of a Silicon Valley venture capital firm and co-founder of the early web browser Netscape.