With the midterms over, Washington returns to its regular programming: Russia. Trump critics should consider the risk of betting their political fortunes on special counsel Robert Mueller.

The Mueller probe has lost its political potency, as Democrats acknowledged on the midterm trail. They didn’t win House seats by warning of Russian collusion. They didn’t even talk about it. Most voters don’t care, or don’t care to hear about it. A CNN exit poll found 54% of respondents think the Russia probe is “politically motivated”;...