It's the magnitude of the Dem victory in Virginia that should worry Republicans:

"Democrat Ralph Northam was elected governor of Virginia Tuesday by an unexpectedly large margin of nine percentage points. He won more votes than any previous candidate for Virginia governor."

"Voters came out in droves. They braved the rain and the cold to send a message to President Trump. The results across the country represent nothing less than a stinging repudiation of Trump on the first anniversary of his election."

“Ed couldn’t escape being a proxy for Trump, which killed him,” added Tom Davis, the former GOP congressman who represented Northern Virginia. “It’s a huge drag on the ticket,” he told Paul Schwartzman. “It motivated the Democratic base. Democrats came out en masse in protest. This was their first chance to mobilize the base."
https://www.washingtonpost.com/news/powerpost/wp/category/the-daily-202/