Democrat Jennifer Wexton tours through Loudoun County visiting the local fall festivals on Oct. 6. (Cal Cary/For The Washington Post)

REP. BARBARA Comstock, a two-term Republican who represents Northern Virginia’s 10th Congressional District, including Loudoun County, is a political survivor. As a state lawmaker, she barely retained her House of Delegates seat after voting against a bipartisan transportation funding bill critical to improving the region’s clogged roads. In Congress, she managed to hold on to her suburban swing district in 2016 even as her constituents voted heavily for the Democrat, Hillary Clinton, for president.

We endorsed her that year, despite disagreements on policy, mainly on the strength of her promise of independence: Ms. Comstock was among a handful of GOP incumbents who said, after hearing Donald Trump boasting about his sexual predations on the “Access Hollywood” tape, that she would oppose him; she called his conduct “obscene.”

Unfortunately, her promise has turned to dust; on critical issues, she has been a reliable, often unquestioning foot soldier in the president’s ranks of Republican loyalists. That disappointment, and the clear and convincing competence of her Democratic rival, state Sen. Jennifer Wexton, inform our decision this year to support Ms. Wexton.

A former prosecutor, Ms. Wexton gained respect in the state Senate, where she has served since 2014, by pushing no-nonsense bills to empower victims of revenge porn to sue their tormentors; toughen regulation of day-care centers; and ease access to medication that prevents opioid overdose deaths. She would be an advocate for gun safety; Ms. Comstock has an A rating from the National Rifle Association, which has lavished money on her.

Ms. Wexton is no partisan bomb-thrower; she rejects talk of impeaching Mr. Trump as “drastic.” She does favor subjecting administration officials to scrutiny when conflict-of-interest concerns have been raised, a responsibility Republicans have abdicated.

Ms. Comstock made her name as a young Republican congressional staffer investigating corruption in the Clinton administration; now, under a scandal-ridden Republican administration, she is mum.

Weeks after the administration began systematically separating migrant families, Ms. Comstock bemoaned the “broken immigration system,” but she subscribed to the president’s phony line that he needed emergency legislation to halt the humanitarian outrage. In fact, after an uproar, Mr. Trump stopped it with the stroke of a pen. When her vote could have made a critical difference in forging a legislative solution allowing hundreds of thousands of “dreamers” to remain legally in the United States — the country where they grew up and went to school after being brought here as children — she kept silent as a group of braver Republican lawmakers tried to broker a fix.

Ms. Comstock, who has voted with the president 98 percent of the time, points to a handful of instances where she has strayed. But in a district with tens of thousands of federal workers, it took scant political courage to oppose the administration’s move to freeze their pay. On most other issues — whether backing a tax bill that inflates the national debt while exacerbating wealth disparity, or disdaining efforts to combat climate change — her arguments are unpersuasive.

Ms. Comstock is energetic, but energy deployed in service of bad policy and the Trump administration’s ethical morass is no virtue. Ms. Wexton would be a breath of fresh air.