ST. PAUL — A congressional rematch in northeastern Minnesota is testing whether Democrats can hang onto a seat they have held almost exclusively for decades.

Rep. Rick Nolan is on his heels again in a race against Republican challenger Stewart Mills, whom Nolan narrowly defeated in 2014. Fueled by more than $12 million in attack ads from outside groups, it’s become the second-most expensive congressional race in the country.

On paper it should be an easier election for Nolan with a presidential turnout surge favoring Democrats. But in a district that’s grown more conservative through redistricting — as evidenced by Republican presidential nominee Donald Trump’s strong showing in March caucuses — nothing is guaranteed anymore. With presidential politics focused elsewhere, Vice President Joe Biden is set to host a Duluth rally Friday.

“We are treating this as a pure tossup,” Mills said. “We’re not starting at zero. We’re picking up exactly where we left off.”

The 8th Congressional District’s sheer size — larger than West Virginia — is matched only by its diversity. It spans from urban Duluth’s liberal voters and old-school Democratic iron mining towns to lakes country and conservative Twin Cities suburbs, with tiny townships scattered in between.