BEFORE SHE entered politics, Valerie Ervin (D-Silver Spring), the Montgomery County Council president, spent years representing blue-collar workers as a union organizer in hard-scrabble places. Unsurprisingly, organized labor, including groups allied with the main county workers union, was among her biggest campaign donors. Now that same union has launched a cowardly campaign of defamation against Ms. Ervin, suggesting — without a shred of evidence — that she is on the take.

The attack has been mounted by the union representing 5,000 clerical, administrative and general workers in Montgomery, which goes by the unwieldy name of United Food and Commercial Workers Local 1994 Municipal and County Government Employees Organization. MCGEO, as it’s known, and its leaders — Executive Director Bob Stewart and President Gino Renne — are evidently bitter that Ms. Ervin, whose election they take credit for, has turned out to be an independent thinker.

Rather than continuing to rubber-stamp unaffordable benefits, unsustainable perks and an inequitable arbitration system that have bled Montgomery’s budget, Ms. Ervin, joined by nearly all her council colleagues, has tried to restore some balance, and some sanity, to county finances. In fact, they’ve had no choice: There is little support for further tax increases in what is already one of the most heavily taxed localities in the region.

Nonetheless, the union, seeing Ms. Ervin’s shift as a betrayal, has unleashed a despicable smear campaign, accusing her of corruptly colluding with a developer to clear the way for a new Wal-Mart to be built in Aspen Hill. In public comments and on the Internet, the union suggests that Ms. Ervin stands to personally profit from “a special deal” with Wal-Mart.

MCGEO’s assertions are ugly, dishonest and absurd on their face. For starters, there’s not a speck of proof behind them. Ms. Ervin hasn’t even said that she would support Wal-Mart, which would require rezoning approval. In fact, she’s pushed legislation requiring Wal-Mart to negotiate with neighbors over what amenities it would pay for before the project gains final approval.

MCGEO itself, along with its affiliated unions and political action committee, has given Ms. Ervin some $15,000 during her races for elective office. By contrast, she received just $1,200 or so in the same period — of a total of $269,000 — from principals in the Lee Development Group, which is seeking rezoning for the new Wal-Mart. Wal-Mart itself has not contributed to Ms. Ervin’s campaigns.

Despite that, MCGEO, in an online attack campaign called TheRealValerieErvin, asked: “How much does [sic] Valerie Ervin, the Lee Development Group and Wal-Mart stand to make from their special deal?” And, in a slanderous remark to, Mr. Stewart, the union head, accused her of a “pay-to-play attitude.” He offered not one fact to substantiate his accusation.

Ms. Ervin says that she has discussed possible legal action with the county attorney in response to the union’s campaign, which she believes is aimed at intimidating the entire council. Whether or not a lawsuit is the right move, there’s no doubt that the union’s tactics are beneath contempt, and a discredit to the honest, hardworking county employees the union purports to represent.