POLITICIANS ARE used to getting calls from special-interest groups lobbying for support on an issue or help in getting a favored candidate elected. No big deal; such leverage is part of the rough-and-tumble of U.S. politics. But the calls that some Montgomery County officials and activists received from the county’s powerful teachers union, seeking to undermine support for a candidate not endorsed by the group, has them thinking that a line may have been crossed.

At issue are phone calls by Jon Gerson, chief political strategist for the Montgomery County Education Association (MCEA), in which his interlocutors say that he threatened to withhold the group’s political support from anyone backing county school board hopeful Rebecca K. Smondrowski. “Support her, and you can just forget about ever getting our support” is how one activist characterized the message from Mr. Gerson. Ms. Smondrowski, whom we have endorsed, is a candidate for the Montgomery County Board of Education in Tuesday’s contest, which will include two candidates backed by the MCEA, Fred Evans and Jeanne Ellinport.

Included in the effort to siphon support from Ms. Smondrowski, sources told us, were personal slanders. “I don’t recall” was Mr. Gerson’s initial response when we asked him about them. When pressed, he said that he doesn’t believe that the conversations occurred in the way described to us, arguing that the credibility of those who insist on anonymity must be doubted. We, too, wish that those who told us of their encounters with Mr. Gerson would agree to be named, but the teachers union has such an outsize role in Montgomery politics that few dare to cross it.

Montgomery County teachers work hard, perform superbly — it’s one of the best systems in the country — and deserve active union representation if they so choose. But there’s a distinction between active and overweening, and the union’s lopsided power in the county doesn’t serve anyone well.

Union-endorsed candidates to the Montgomery County Council, the school board and the General Assembly sometimes end up toeing the union line to the detriment of other county priorities or county workers. We’ve written before how the union, in a reversal of normal practice, pressured candidates for contributions to help finance its campaign activities. And now this, an underhanded whispering campaign against a well-qualified candidate who has been endorsed by a number of state and local officials, including Maryland Comptroller Peter Franchot (D), Del. Benjamin F. Kramer (D-Montgomery) and state Sen. Jamie Raskin (D-Montgomery).

The union has the right to pick candidates whom it believes will best advance its interests. It should make its case for those candidates on their merits.