Marc Elrich, center, in Silver Spring in February. (Bill O'Leary/The Washington Post)

BALANCE HAS been the hallmark of leadership in Montgomery County for the past 12 years under County Executive Isiah Leggett (D), who is retiring. That very quality has eluded the front-runner to succeed him this fall.

We refer to Marc Elrich, the Democratic nominee for county executive in Maryland’s most-populated locality, a jurisdiction where Democrats outnumber Republicans by 3 to 1. Paradoxically, Mr. Elrich is both a well-known and an uncertain commodity — well-known for his leftist, anti-development positions over the past 12 years on the county council; uncertain in terms of abilities as an executive .

Among the questions about Mr. Elrich is one he raised himself by pledging to invite the president of the largest county employees union into interview and hiring deliberations “for any and all department heads” in the county. That is an extraordinary promise, even for a pro-labor politician — and one without precedent in any area jurisdiction, as far as we can ascertain. It is also unwise.

Mr. Elrich, backpedaling, now says labor leaders — including the head of Montgomery’s largest public-sector union, UFCW Local 1994 Municipal County Government Employees Organization — would be welcome to join him in interviewing candidates to run departments, but not in hiring decisions. Either way, the practice could leave top administrators beholden to the very people they manage. It assumes that organized labor’s interests dovetail neatly with those of Montgomery’s nearly 1.1 million residents, and with the goals of efficient government Mr. Elrich says he supports.

As anyone, Democrat or Republican, who has run a state, county or municipal government can attest, that is not the case, to put it mildly. Collective bargaining is always contentious: Unions are duty-bound to press for the most generous package they can extract, in this case from taxpayers. As county executive, Mr. Elrich’s job would be different: to balance a fair deal for workers, whose pay and benefits consume 80 percent of county spending, with other stated — and expensive — priorities: expanded early-childhood education, more affordable housing, and a bus rapid-transit network.

That’s not the only tricky tightrope Mr. Elrich has strung for himself. Mindful that Montgomery faces tight fiscal and revenue restraints — “the truth is we don’t have any money,” he told Bethesda Magazine — he has cast doubt on the county’s long-standing goal of boosting its reserve fund to 10 percent of revenue. However, Mr. Elrich himself voted for that target, which Montgomery is on track to reach by 2020. And any retreat would jeopardize the county’s AAA bond rating, drive up borrowing costs by tens of millions of dollars annually and deal a severe blow to the county’s reputation, as well as its ability to support everything from public schools to police.

Avoiding such pitfalls will require the kind of balance that Mr. Leggett has exemplified.