Jeff C. McKay won the Democratic contest for chairman of the Board of Supervisors. (Michael S. Williamson/The Washington Post)

IN FAIRFAX COUNTY, one of the nation’s biggest, richest, most diverse and most economically vibrant suburban localities, the results in this week’s Democratic primaries for state and local office were a victory not just of experience but also of substance and issue mastery over ideology and identity politics. That matters, because Fairfax — population 1.2 million, a third of them foreign-born, and with half of its 100,000 elementary school students speaking one or more of 182 foreign languages — is too big, too complex and too important for the future of the national capital region to be governed by novices.

The county, having been politically contested for decades, is now solidly Democratic, so the winners of the June 11 party primaries are heavy favorites in the fall general election. For the most part, they are deeply knowledgeable candidates whose liberal outlook is tempered by pragmatism.

First and foremost, they include Jeff C. McKay, winner of the Democratic contest for chairman of the Board of Supervisors, the county’s governing body, whose nearly quarter-century of service includes stints as the Lee District supervisor, a position he’s held since 2008, and as chief of staff to his predecessor in that job, Dana Kauffman. Mr. McKay — strategic-minded, detail-oriented and thick-skinned — turned back three challengers with less experience and familiarity with policy, including one who launched his campaign by leveling anonymous, apparently unfounded allegations against Mr. McKay. It’s a credit to Fairfax voters that they handed Mr. McKay what turned out to be a comfortable victory.

Four other winners in the primaries are also skilled and well-informed heavyweights. Walter L. Alcorn, who won the Hunter Mill District race, is among the county’s leading experts on land use at Tysons Corner, whose long-term development plan he was instrumental in devising. Rodney L. Lusk, the primary winner in Lee District (where he would replace Mr. McKay), has functioned as what amounts to Fairfax’s top salesman in his capacity as national marketing director for the county’s Economic Development Authority. Dalia Palchik, the Providence District primary winner, has been a community leader as a member of the county school board, to which she was elected in 2015, and on the Tysons Regional Chamber of Commerce. And the Braddock District winner, James R. Walkinshaw, is a longtime top staffer for Rep. Gerald E. Connolly (D), who chaired the county Board of Supervisors before he was elected to Congress.

In several races, the winners defeated candidates who channeled the Democratic Party’s insurgent liberal wing by attacking the status quo. What distinguished those who prevailed was not that they lacked progressive credentials, but their depth of understanding of policy and practice in a 400-square-mile jurisdiction facing a paucity of affordable housing; aging infrastructure; a widening gulf between haves and have-nots; and, in some communities, tensions over development. That depth of understanding carried the day in a county known for cautious, calibrated good government.