YOU'D THINK someone would have known better: That has to be the first reaction to news of public television stations swapping contributor lists with the Democratic National Committee. These are stations that rely on public funding and profess impartiality. Staying away from politics is -- should have been -- a matter of common sense.

Yet two months ago, the Boston Globe reported that WGBH, the local public station, had traded lists with the Democrats. The station first portrayed that event as a one-time mistake, then had to acknowledge repeated such incidents. Other stations, including Washington's WETA, reported that they, too, had swapped lists with political organizations, conservative as well as liberal. San Francisco's station traded lists with the campaign of Democratic Sen. Barbara Boxer. No doubt other examples will emerge; public broadcast officials say about three-dozen stations have used "list brokers" whose business is helping nonprofits swap donor lists.

The predictable excuses have been trotted forward, but happily not from the leaders of public broadcasting. The heads of the Public Broadcasting Service, National Public Radio, America's Public Television Stations and the Corporation for Public Broadcasting promptly pledged to work with Congress to ensure that such exchanges no longer take place. You could argue, given congressional anger, that the broadcast executives didn't have much choice. But they seem to understand that not only their funding is in jeopardy. As a spokesman for Rep. Billy Tauzin said, "It undermines the faith in public broadcasting."

Some executives seem to have understood all along; 31 of the major stations have a policy prohibiting the exchange or "rental" of donor lists with anyone. That has to be right. The stations may now produce evidence to dispute the impression of liberal bias -- to show that they shared with Republicans, too. That won't reassure us much. When people write checks to support Big Bird or Jim Lehrer, they shouldn't be opening themselves up to political solicitations as a result.