– Residents in Duluth, Brainerd and the Iron Range: Get ready for a sustained bombardment of political advertising.

The Eighth Congressional District race between Democratic Rep. Rick Nolan and GOP challenger Stewart Mills has now drawn the largest flood of outside cash in the country.

As of Thursday, more than $9.6 million had poured into television advertisements and mailers in the northern Minnesota district, which has seen congressional representation swing back and fourth between parties over the past decade.

The intense spending highlights that both sides believe the district is winnable and could become critical in the emerging battle for control of the U.S. House. The nation-leading deluge of cash is also remarkable because advertising there is relatively cheap, compared to battlegrounds in New York or Southern California.

Leaders of these outside groups don’t expect their efforts to let up, and are vowing to spend even more in the final 18-day sprint to Election Day.

“I don’t think they could put any more here, unless they add an hour to the day,” said Cynthia Rugeley, a University of Minnesota Duluth political scientist.

Real Clear Politics moved Minnesota’s Eighth to a “pure toss up” race this week. The Rothenberg & Gonzales Political Report handicaps Nolan as “Democrat favored.”

For Mills and Nolan, both frantically crisscrossing the district in buses and RVs to meet voters, the flood of outside money means they don’t have to spend as much on advertising.

“It’s a neck-and-neck pure coin flip,” Mills said Thursday. “All we have to do is park our bus, people are coming up to us, wanting to take pictures, shake hands. It’s a really good sign.”

Mills cited GOP presidential nominee Donald Trump’s popularity in the district and called many of President Obama’s policies, including the Affordable Care Act, unpopular among district voters. Mills also cited his A rating by the National Rifle Association, the influential gun-rights group.

Even so, the Mills Fleet Farm heir has had to put a lot of his personal fortune into the race. He has loaned his campaign $1.7 million so far and had $328,000 cash on hand at the end of September.

Nolan, who is running for his third term this time around (he was in Congress in the 1970s), beat Mills by just 1.4 points two years ago. Nolan is pushing a message of helping the middle class through higher taxes on the wealthy. He also says he will protect jobs on the Iron Range by continuing to fight for tariffs on countries illegally dumping steel. He acknowledged Trump’s popularity in the district.

“A lot of polling is showing a fair number of people voting for Trump and Nolan,” said Nolan, who had $669,074 in cash in his campaign. “I’ve been an effective member of Congress. I’m confident I’m going to win it.”

These independent expenditure groups, including super PACs, are allowed by law to raise and spend cash on get-out-the-vote operations, mailers and advertising for or against federal candidates. These expenditures are not considered campaign contributions; in fact, the groups are explicitly prohibited from coordinating with the official candidate campaigns.

Two big factors are fueling the advertising arms race in northeastern Minnesota.

One is the popularity of Trump among voters in the district. Most Republican and Democratic internal polls put Hillary Clinton and Trump within a few percentage points of each other in the region.

Nolan’s seat is particularly sweet to Republicans since they don’t have many opportunities to pick up House seats this year, due in part to their massive majority in the House (246 GOP to 186 Democrats). Trump’s lagging polls in recent weeks also could be hurting their chances in other battleground districts.

The other factor for the high spending is the sheer size of the district and the expense required to reach voters. Outside groups — like former GOP Sen. Norm Coleman’s super PAC — that want to blast an ad criticizing a rival candidate must buy time in both Duluth and the Twin Cities media markets to reach everyone.

Coleman’s outside groups, the American Action Network and the Congressional Leadership Fund, have pledged $4.5 million to help Mills. The Democratic-affiliated House Majority PAC and the Democratic Congressional Campaign Committee are stepping up, as well. House Majority PAC pledged $2.2 million this week to help Nolan.

Rugeley, the professor, said she wonders if at some point the money starts working against itself.

“I’m a political junkie, but I know toward the end … people just get tired of it. They [ads] stop having any effect because people aren’t listening.”