Democrat Patty Schachtner scored a 9-point victory in a special election in Wisconsin's State Senate District 10 last night, flipping a district that had been held by the GOP since 2000 and that President Trump won by 17 points in 2016, per The Huffington Post.

Why it matters: It's a massive swing to Democrats in a Republican-heavy area ahead of November's midterms — and elected GOP officials are spooked:

And that's not all, per Daniel Nichanian at the University of Chicago, there were three other worrying results for Republicans in state special elections last night — though they still managed to hang onto the following seats in these GOP-heavy areas:

• South Carolina's House District 99 had a 15% net swing to Democrats.
• Iowa's House District 6 had an 18% net swing to Democrats.
• Wisconsin's Assembly District 58 had a 25% net swing to Democrats.

One more thing: Republicans maintain their stronghold in Wisconsin. The state is one of 26 Republican trifectas (the governor is Republican, and Republicans have the majority in both the state Assembly and the state Senate). And even with Schachtner’s win, Republicans will hold a 18-14 majority, per the Milwaukee Journal Sentinel.

Go deeper: If these results show on a national scale in November, they could usher in Trump's greatest nightmare.

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