THE DEMOCRATIC primary race to succeed Rep. James P. Moran, who has represented Northern Virginia’s heavily Democratic 8th Congressional District since 1991, is strikingly lopsided. Half a dozen candidates have scrambled to dislodge a smart, substantive, well-financed front-runner, former lieutenant governor Don Beyer. For the most part, they appear to have failed — not because they are all pygmies, but because Mr. Beyer is, simply, an excellent candidate. He would make a first-rate addition to this region’s unusually effective congressional delegation, and we endorse him in the June 10 primary.

In addition to having helped run and expand his family’s eponymous car dealership, Mr. Beyer has built a career in politics, public office and, most recently, diplomacy, which none of his opponents comes close to matching. As lieutenant governor for eight years in the 1990s, he was respected as an effective advocate for Virginia’s emerging high-tech business sector and as a knowledgeable reformer unafraid of immersing himself in the policy weeds on disability, welfare reform, poverty and other issues.

After losing the race for governor in 1997, Mr. Beyer shuttled between business and high-profile Democratic campaigns, working in key fundraising roles for the presidential campaigns of former Vermont governor Howard Dean, John F. Kerry and President Obama. In Mr. Obama’s first term, he rewarded Mr. Beyer by naming him ambassador to Switzerland and Liechtenstein, where he served for four years.

There is no shortage of well-heeled political insiders with resumes like Mr. Beyer’s who project a sense of entitlement or, in ambassadorial roles, no more than a passing familiarity with their briefs. Mr. Beyer, to his credit, projects the opposite. In candidate forums, where most of his opponents flounder when foreign affairs questions are posed, Mr. Beyer is at ease, with a deep and solid grasp of U.S. policy and events in Europe and the Middle East.

He has a similar command of climate change issues, which he says would be his focus in Congress. And while some of his opponents have jockeyed to claim the title as the most left-leaning liberal in the race — and pander to primary voters in the 8th District, one of the most reliably Democratic seats in the country — Mr. Beyer, alone in the field, has a credible chance of working across the aisle to enact legislation, even in Capitol Hill’s shambolic status quo.

Mr. Beyer’s opponents include a pair of state lawmakers (Sen. Adam P. Ebbin and Del. Patrick A. Hope); Alexandria Mayor William D. Euille; and Mark Levine, a radio talk-show host. Mr. Levine, who was once an aide to former representative Barney Frank (D-Mass.), is the only one with knowledge of Congress, but his passion for partisan attack seems more likely to yield more bad blood in Congress (if that’s possible) than results.

Mr. Beyer hasn’t appeared on a ballot in 17 years, but he’s stayed active on issues that have concerned him for years, including teen pregnancy and high school dropouts. Such problems are critical in the district, which includes Alexandria, Arlington, Falls Church and parts of southern Fairfax County. As nearly as any freshman could, he’d be effective in Congress from day one.