IT CAN be easy to forget the spectacular mess that Rushern L. Baker III inherited when, in 2010, he became the top elected official in Prince George’s County, Maryland’s second-most-populous locality. Given his brand of steady, sane, no-drama leadership as county executive over the past eight years, it is equally easy to overlook the extent to which he has succeeded in radically rebranding a locality whose reputation had been so badly stained by public corruption, chaotic governance and crime.

Mr. Baker’s admirable, ambitious record in Prince George’s is the basis on which we support him in Maryland’s Democratic gubernatorial primary, on June 26. In a solid field of primary candidates, which includes a respected state senator, a nationally known civil rights leader and others with impressive résumés, Mr. Baker stands out for having run what amounts to a turnaround project in a county of nearly 1 million residents.

He has lately come under fire from his rivals, including Gov. Larry Hogan, a Republican, for controversies in the Prince George’s school system, one of the nation’s largest, including inflated graduation rates, mishandled sex abuse cases and questionable raises for top aides to the outgoing schools CEO, Kevin M. Maxwell, who was chosen by Mr. Baker.

What gets lost in the hubbub is that Mr. Baker, virtually alone among county executives in Maryland, had the political courage to seek and attain personal authority over public education in the county. His willingness to be politically accountable for reforming a huge system long troubled by poor performance and managerial dysfunction was a gamble. And despite the recent troubles, it paid off to a large degree: Enrollment climbed and test scores crept up as schools offered an enriched menu of academic choices, including expanded full-day prekindergarten and language immersion.

The Democratic primary contest, whose winner will challenge Mr. Hogan in November, has been marked by opportunistic sniping at Mr. Baker’s record with the schools, in most cases by rivals who lack his breadth and depth of experience. Well before he became county executive, Mr. Baker, as a state lawmaker from Prince George’s, was an effective advocate for improving public schools.

After a stint leading a nonprofit focusing on education, he took over the county government eight years ago amid turmoil. For years there had been rumors and reports of the county’s pay-to-play politics. Shabby ethical conduct was rife among sticky-fingered public officials, several of whom thought little of using county credit cards to cover personal expenses. Sweetheart contracts, jobs and deals went to the well-connected, often of dubious qualification. Few people seemed very upset about the status quo.

That all exploded weeks before Mr. Baker took office. Federal agents arrested the man he would succeed, then-County Executive Jack B. Johnson, along with his wife, Leslie, who had been elected to the County Council. As the agents entered their home, Mrs. Johnson, at her husband’s behest, was stuffing nearly $80,000 in her bra and flushing a check for $100,000, obtained as a bribe, down the toilet. Both pleaded guilty and went to prison.

Having promised a new day in Prince George’s, Mr. Baker more than delivered, infusing the county not only with beefed-up ethics rules — his tenure has been mostly scandal-free — but also with new opportunities and a sense of economic vibrancy. He sealed a deal to attract the $1.4 billion MGM resort and casino to National Harbor, generating millions of dollars in tax revenue, thousands of jobs and legions of visitors. He pushed through plans to replace the county’s chronically penniless hospital with a modern medical complex. And he positioned the county as a strong contender to land the FBI’s new headquarters, until the Trump administration pulled the plug on the project.

Crime has plummeted on Mr. Baker’s watch, buttressing investors’ confidence, and last year Whole Foods opened its first market in the county — a milestone in one of the nation’s most populous and prosperous majority-minority jurisdictions.

Mr. Baker says he would focus his energies as governor on education, as well as transit and health care (with a focus on mental health and the opioid crisis). He insists he is prepared to make cuts in other areas of government to further that agenda, especially to free up funds to improve the state’s schools. It’s a clearsighted blueprint, and Mr. Baker, having served nearly a decade in the state legislature, has the allies in Annapolis he’d need to help enact it — though we reserve judgment on a general-election contest including Mr. Hogan.

Among Mr. Baker’s Democratic rivals, the most prominent are Benjamin Jealous, a venture capital executive and former president of the NAACP, and state Sen. Richard S. Madaleno Jr. (Montgomery), a longtime legislator known for his expertise on Maryland’s budget. Both are serious and substantive candidates; neither has undertaken Mr. Baker’s trial by fire in reinvigorating a major locality with urban, suburban and rural areas, or balancing the range of competing interests that make up a county of Prince George’s scale. He is, by a wide margin, the strongest candidate in the primary field.