MOST OF this year’s suburban congressional races in the Washington area are slam dunks. It’s plain as day that incumbent Reps. Don Beyer in Virginia’s 8th District and John Delaney in Maryland’s 6th District — unusually smart, decent and diligent Democrats who turned to politics after impressive careers in business (and, in Mr. Beyer’s case, a stint as an ambassador) — are vastly more experienced and knowledgeable than their Republican opponents, both political novices.

Ditto Maryland State Sen. Jamie Raskin, the Democrat running to succeed Rep. Chris Van Hollen in Maryland’s 8th District. We disagree with the very liberal Mr. Raskin, a constitutional law professor, on some issues, but we admire him for his intellect and prodigious legislative achievements.

As for Mr. Van Hollen, whose command of budgetary and fiscal policy has made him among the most respected Democrats in Congress, he is cruising to what should be a lopsided victory in his U.S. Senate race against a Republican state lawmaker, Kathy Szeliga, who is at sea with federal issues.

For us, the toughest call is in Virginia’s 10th District, which runs from McLean and other close-in suburbs west through Loudoun County to the West Virginia line. There, Rep. Barbara Comstock, a freshman Republican and former political operative, is challenged by Democrat LuAnn Bennett, a real estate executive. Both candidates are up to the job. We endorse Ms. Comstock, one of fewer than two dozen House Republicans seeking reelection who have announced their opposition to Donald Trump’s candidacy.

We did not support Ms. Comstock in the past, during her four years as a state lawmaker and when she ran for Congress, because of her ideological rigidity; she signed a blanket pledge opposing higher taxes and, in Richmond, voted against a once-in-a-generation transportation funding package backed by Republican leaders and key to the state’s economic prospects.

In Congress, where she represents a swing district, she has taken some steps to temper her hard-right conservatism, voting against a government shut-down, for example, and protecting federal funding for Metro when it was threatened by other Republicans. The word “Republican” is absent from her campaign website; the word “bipartisan” appears throughout.

We disagree with Ms. Comstock on an array of issues, including her opposition to gun-safety measures and her vote to defund Planned Parenthood, which provides critical health services for millions of women. Yet her withering assessment of Mr. Trump, and some of her votes in Congress, suggest she might bolster a conservative core of sane House Republicans who could face down the party’s ascendant dead-enders. In the wake of its disastrous presidential nomination this year, the Republican Party will badly need elected officials willing to govern, not just wage partisan warfare. If Ms. Comstock can play that role, and champion vital regional priorities such as Metro, it would be invaluable. She has given some signs, if not yet proof, that she is willing; our endorsement is a calculated hope that after wrapping herself in a bipartisan banner, her actions will match her rhetoric.