EAU CLAIRE, Wis. (AP) — For a brief moment this week, the fierce competition for swing voters in swing-state Wisconsin converged on the tarmac of the tiny Chippewa Valley Regional Airport.

Minutes after Vice President Kamala Harris landed with her newly minted running mate Tim Walz for their first campaign stop in the state, Republican vice presidential nominee JD Vance arrived. He walked across the tarmac to check out Air Force Two, just missing Harris.

The close encounter of the political kind could be written off as a coincidence if it happened anywhere other than Wisconsin, one of a small number of states that will not only determine the winner of the presidential race but could also shape the balance of power in Congress. But it sent a much louder signal that both parties understand the importance of a region that could tip the balance of power in more ways than one.

The western Wisconsin congressional district where Harris, Walz and Vance all campaigned Wednesday is one Democrats hope to flip this year as they look to cut into the narrow GOP majority in the House. It’s also crucial to U.S. Sen. Tammy Baldwin’s reelection bid, a must-have for Democrats if they are to hold onto the Senate.

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The small airport in Eau Claire in Wisconsin’s 3rd Congressional District is far from the Democratic urban strongholds to the east and the deep-red rural swaths of northern Wisconsin. But the area is essential for both Harris and former President Donald Trump in one of the “blue wall” states, along with Pennsylvania and neighboring Michigan, that both parties say they need to win to secure the White House.

The battle lines will become clearer Tuesday in a competitive Democratic primary for the congressional seat, which is currently held by Republican Rep. Derrick Van Orden, one of Trump’s loudest backers.

“The 3rd Congressional District is a purple district in a purple state,” said Democratic U.S. Rep. Mark Pocan, from Wisconsin’s neighboring congressional district to the east. “How it votes will very likely indicate how it goes statewide for Vice President Harris.”

Republican U.S. Sen. Ron Johnson called it “a bellwether district in a bellwether state.”

The diverse district has historically been drawn to moderates like Democrat Ron Kind, who held the congressional seat for 26 years before retiring in 2022. Before him, moderate Republican Steve Gunderson held the seat for 16 years. Democrat Barack Obama won the district twice in his White House races, but Trump won there in both 2016 and 2020 as rural areas nationally became more conservative.

Johnson said Trump’s presence on the ballot helped him win in 2016, boosted by the former president’s appeal to the working class and farmers. But that doesn’t mean the district is reliably Republican.

“The people of the 3rd District are independent thinkers who don’t like to be pigeonholed one way or the other,” said Republican Brian Westrate, who grew up and lives in the district. “They don’t like people telling them who they are or what they believe. They can’t be taken for granted, one way or the other. You have to earn their votes.”

Republicans have done a better job addressing the issues they care about, said Westrate, who serves as treasurer for the state Republican Party.

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“They are people who like to get out there during the nine-day gun hunting season,” he said, “and that’s as much of a religious experience as Christmas.”

With the Mississippi River and the border with Minnesota to the west, the district includes the rolling hills of the Driftless Area and Chippewa Falls, home of Leinenkugel beer. From the Illinois border, it stretches 250 miles north past Prairie du Chien, known for its Cabela’s outdoor gear distribution center and 19th-century riverside historic sites.

Harris and Walz campaigned on the western edge of the district in Eau Claire, home to a University of Wisconsin campus, about an hour’s drive east of the border with Minnesota, where Walz has served as governor since 2019.

Harris is counting on his appeal to translate across state lines. It did in reverse Wednesday, when many of the more than 12,000 people her campaign says attended the rally came in from Minnesota.

A strong showing by Harris and Walz in western Wisconsin would take pressure off Democrats in Milwaukee and Madison to deliver huge margins to counter Republican strongholds in the suburbs and rural parts of the state, said Anthony Chergosky, a University of Wisconsin-La Crosse political science professor.

“They don’t necessarily have to win this congressional district,” Chergosky said of Democrats. “But they can’t get blown out.”

Pocan has argued Democrats didn’t spend enough money to win the open 3rd Congressional District race in 2022. This year, as the Republican Van Orden seeks reelection, Democrats are making the race a priority. The Democratic House Majority PAC in July added the race to its list of 31 target seats as part of a $24 million ad buy.

Unlike Kind and his moderate Republican predecessor, Van Orden aligns with the far right of the Republican Party. Democrats think that opens a door for them.

Van Orden attended the Jan. 6, 2021, rally before a mob of Trump supporters stormed the U.S. Capitol. Since winning the election in 2022, he was chastised by leaders of both parties for yelling at high school Senate pages in the U.S. Capitol, shouting “Lies!” during President Joe Biden’s State of the Union speech this year and getting into a scuffle with a liberal activist at the Republican National Convention in Milwaukee.

Van Orden’s campaign did not return a message seeking comment.

The Democratic congressional primary, which will be decided Tuesday, also will test the strength of Democrats outside the party’s urban hubs in Milwaukee and Madison. Each of the three Democratic candidates has taken a different approach, offering a kind of messaging laboratory for how best to connect with swing voters.

“There’s a very interesting conversation ongoing in the Democratic Party right now about what’s going to sell in this part of the state,” Chergosky said.

Rebecca Cooke is a former political fundraiser and former small business owner who ran and lost in the 2022 Democratic primary. She is running as a political outsider, a position her challengers have questioned given the years she spent working as a political activist and fundraiser in Washington. Cooke touts her upbringing on a dairy farm and management of a nonprofit aimed at helping women-owned businesses, and she continues to work as a waitress.

State Rep. Katrina Shankland, who has been in the state Legislature since 2013, argues that she’s the candidate with the most experience in office. She has the backing of more than 18 labor unions and touts the more than 200 bills she’s supported that have been signed into law by either the current Democratic governor or his Republican predecessor.

The third contender, Eric Wilson, is a political newcomer trying to position himself as the most liberal candidate, touting his support for “Medicare for All” and the “Green New Deal,” a set of policy proposals on the environment touted by liberals in Congress.

At the end of June, Cooke had more than $1 million cash on hand compared with about $342,000 for Shankland and just $42,000 for Wilson. Van Orden, meanwhile, had $2.3 million on hand ahead of the general election.