(Jahi Chikwendiu/The Washington Post)

IN A county as sprawling and complex as Fairfax — more than 1 million people, a third of them foreign-born , in a 400-square-mile locality with a budget larger than that of several states — it’s a big deal when half the local governing body turns over. That’s what the county’s Board of Supervisors is facing, with four of the board’s 10 district supervisors, as well as the chairman , leaving their positions after decades of combined experience.

With the county having trended Democratic, most of the action to replace them is in that party’s primaries June 11; there are no contested Republican primaries for the board. The eventual winners will face an array of challenges: managing land use; improving transportation; preserving and expanding affordable housing; revitalizing older and poorer neighborhoods; and maintaining the county’s status as a magnet for companies and jobs to sustain and improve services, including for one of the nation’s biggest and best public school systems.

We’ve already endorsed an exceptional candidate in the primary race for the chairmanship, Jeff C. McKay , the current Lee District supervisor. The following are our endorsements in the contested Democratic primaries for district supervisor.

In the Braddock District, James R. Walkinshaw , longtime chief of staff to Rep. Gerald E. Connolly (D) , is a solid candidate facing nominal opposition in the primary to succeed departing Supervisor John C. Cook, one of just two Republicans on the board. In addition to Mr. Walkinshaw’s experience on Capitol Hill, he is well versed on county issues, especially transportation, having also served on Mr. Connolly’s staff when he chaired the Board of Supervisors more than a decade ago.

In the Hunter Mill District, where Supervisor Catherine Hudgins is retiring, the best candidate is Walter L. Alcorn, a formidable land-use expert widely admired for his work on the planning commission overseeing the county’s long-term blueprint to transform Tysons into a more walkable, livable downtown. No candidate is better equipped than Mr. Alcorn, who served on the county planning commission for 16 years, to negotiate the thorny debates over development in Reston, the district’s hub.

In the Lee District, the strongest candidate in a competitive field is Rodney L. Lusk, whose broad and deep expertise in local matters extends to land use and, in recent years, working to attract employers to Fairfax as the national marketing director for the county’s Economic Development Authority, a key job that would position him to help lure firms to the Route 1 corridor.

In the Providence District, the retirement of Supervisor Linda Q. Smyth has triggered a scramble among five primary candidates. Several are plausible; the most impressive is Phillip A. Niedzielski-Eichner , whose array of leadership experience in Fairfax, over three decades, is nearly unparalleled. It features stints on the county school board, including as chairman; the Park Authority board; and with the Ad Hoc Police Practices Review Commission, on which he led efforts to reform the police department’s use-of-force policies in the wake of an unjustified police shooting. He is currently the Providence District representative on the county Planning Commission. Few candidates for the board of supervisors can match Mr. Niedzielski-Eichner’s range of wisdom and knowledge.

Democratic Supervisors John W. Foust (Dranesville), Kathy L. Smith (Sully) and Daniel G. Storck (Mount Vernon) are running unopposed , while Supervisors Penelope A. Gross (D-Mason) and Pat Herrity (R-Springfield) face only general election challengers.