A record amount of outside spending has flowed into U.S. House races in Minnesota, including the 2nd Congressional District.

The rematch between democrat Angie Craig and Rep. Jason Lewis is one of 40 races nationwide where outside spending has outpaced that of candidates, an analysis from the Center for Responsive Politics shows. Close to 95 percent of the $8,404,970 reported Nov. 5 has gone into ads supporting or attacking either candidate, said Andrew Mayersohn, one of the center's analysts. The interest from groups such as 501c organizations, political action committees and super PACS reflects the district's status as a battleground for Democrats and Republicans hoping to snag majority of the U.S. House.

The seat, which Lewis won in 2016 by just under two percentage points, has been held by a Republican since 2000. But the district tends to support both Democrats and Republicans for various offices each year. It went for President Donald Trump over Hillary Clinton in 2016 by about 1.2 percentage points, and Barack Obama by one-tenth of a percentage point in 2012.

"The bottom line is there just aren't that many competitive districts across the country, so outside groups are focusing on races that have the most potential," said Kathryn Pearson, a University of Minnesota political science professor.

Out of the four competitive districts in Minnesota, the 2nd Congressional ranks lowest for outside spending. The 1st District has had the most, with about $13,798,070 as of Nov. 5.

Full calculations of outside spending won't be available until final reports are filed after the election. Still, this year's groups have spent at least $2 million more than they did in Craig's and Lewis' first square-off in 2016. Prior to that, outside spending rarely went above $50,000 in this district.

The unprecedented amount of outside spending has resulted in a record amount of negative ads for voters, since most candidates leave the attack ads to outside groups, Pearson said.

Of the roughly $8.5 million going into the race, $5,343,481 has gone into ads that either opposed Lewis and supported Craig, while $3,065,489 has been spent spent to support Lewis and oppose Craig.

The main PAC for each party - the National Republican Congressional Committee and the Democratic Congressional Campaign Committee - are the top spenders. Out of all the races in the country, Minnesota's 2nd Congressional District ranked sixth on the NRCC's list for spending, and 15th on the DCCC's list for most spending, per the Center for Responsive Politics.

"That tells you a lot about this race. The parties are invested. These are the two big groups that need to make decisions about how to allocate resources for every race in the country. The parties clearly see this race as an important race," said Chris Chapp, a political science professor at St. Olaf College.

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Experts say voters should be aware of the outside spending for a few reasons.

First, it's not always easy to tell who exactly is paying for advertising, Chapp said. While direct contributions to candidates are documented and have certain limits set by the Federal Election Commission, there are less regulations for outside spending. These groups are not allowed to coordinate with campaigns, but they can raise virtually unlimited amounts of money, sometimes without disclosing where they get it from, he said.

While PACs and super PACs both have some disclosure methods, 501c organizations are not required to reveal donors. And sometimes, 501c organizations will contribute their money to a PAC or super PAC, Mayersohn said.

"You've got 501c organizations spending a lot of money in these elections and we don't know who's behind those donations," Chapp said.

About 10 percent of the money in the 2nd District has come from 501cs.

Other than disclosure questions, the prominent concern is that candidates could become beholden to the interest groups spending money to help them - though this is very difficult to prove, Chapp said.

"From a political science perspective, it's really hard to know. Is some super PAC influencing a candidate to vote a certain way, or is a super PAC giving money to a candidate who they know will support their views anyway?" he said. "So it's difficult to know ... if a politician is being bought and paid for. It's always a concern."

Chapp noted that a diverse range of interests are represented by the groups, including the environment, reproductive rights and gun control.

"That might bring some comfort to people who are concerned that there is one issue and that's the only thing a candidate cares about," he said.

Still, he said the main impacts of outside spending are yet to be proven with studies.

"There's always a question, does all this spending actually matter? And my answer is, we're going to find out," he said. "The political science literature is a little bit mixed on how much money actually influences elections. It's one of these things that's really hard to study."