Polls have closed and the battle between Republican Troy Balderson, 56, and Democrat Danny O’Connor, 31, will be over soon.

Why it matters: Ohio's 12th district should be a dream for the GOP — it’s solidly Republican and overwhelmingly white. But it's considered competitive for the first time in decades. If Republicans can’t pull of a win in a race like this they should be worried about what’s in store for them on Nov. 6.

What to know: A Democrat hasn’t represented this area since 1980. President Trump won the district by 11 points.

• Republicans have a two-to-one voter registration advantage in the 12th district.
• Republicans and outside GOP groups have out-spent O'Connor 5-to-1 to help Balderson.
• There are 79 GOP districts more competitive than OH-12, where Trump got less than the 53% he earned in this district in 2016.

If Democrats win, it’s another sign that their base is turning their anti-Trump energy into showing up to vote. And it’ll add to another unlikely victory after Democrats flipped seats in special elections in Alabama and Pennsylvania.

If Republicans win, they’ll sleep a little easier in the weeks leading to November knowing they still have a stronghold on the Midwest. They won’t be able to replicate their expensive playbook in every competitive race, but they’ll have a few lessons — like where and how to spend their money — that’ll come in hand throughout the country.

Let’s go to the data … FiveThirtyEight reports, since 2017 "the margins of federal special elections have shifted [toward Dems] by an average of 16 percentage points." If Ohio follows that trend, O’Connor would win by 2 points (a recent poll showed him leading by 1 point).

Bottom line: Republicans can’t afford to lose the open seat in Ohio’s 12th district to Democrats.