The League of Women Voters, perhaps best known for its dry, down-the-middle voter guides, is running a hard-hitting TV ad that slams US Sen. Scott Brown for voting to shortchange the environment. “Scott Brown should protect the people, not the polluters,” says the ad’s tagline.

Brown is firing back. In an op-ed piece this week in the Boston Herald, Brown condemns the “negative attack ad” and the group that sponsored it. “It’s shocking that a supposedly nonpartisan group like the League of Women Voters has engaged in this demagoguery,” he says.

It’s a fascinating political fight, and media outlets are flocking to cover it. But it’s also another example of how the press tends to cover the sideshow rather than the main attraction.

Brown cast his controversial votes early last month as the Senate took up amendments that would have stripped the Environmental Protection Agency of its power to regulate greenhouse gases. One of the amendments was defeated on a partisan 50-50 vote. In supporting the amendment, Brown sided with every other Republican in the Senate except one, Susan Collins of Maine, who voted against it.

Local news coverage of Brown’s votes was pretty skimpy. The Globe ran a short Associated Press news story inside the newspaper while the paper’s editorial page took Brown to task. But most other media outlets ignored the story, which is partly why the League of Women Voters decided to jump into the fray.

“This vote for Sen. Brown is out of step with his constituency,” says Elisabeth MacNamara, the Atlanta-based president of the League.

Brown catapulted into office with the help of national conservative support and yet represents a heavily Democratic state. With his votes, he’s always walking a fine line.

His political predicament is not unlike that of Mitt Romney, who embraced universal health coverage as governor of Massachusetts yet has distanced himself from that legislation in his runs for the presidency. Like Romney, Brown supported the Massachusetts reform law, voting for it as a state senator, but he now opposes a national version at the federal level. Similarly, he voted as a state senator to have Massachusetts join with other Northeastern states in setting up a cap-and-trade system for carbon emissions, yet now opposes a carbon tax in Washington.

In an op-ed article in the Cape Cod Times, Brown said he feels it’s not right to give an “unelected bureaucracy” at the EPA the power to regulate carbon emissions. “What Bay State businesses don’t need is the federal government arbitrarily passing down restrictions that would dramatically restrict their potential for growth by saddling them with higher costs,” he wrote.

Brown’s original votes didn’t garner that much media attention, but now that he and the League of Women Voters are mixing it up media outlets are starting to play catch-up. Some of them are writing editorials condemning the League’s “blatant partisanship.”

The League’s MacNamara says her organization is political but nonpartisan. She notes the League is also running an ad condemning an environmental vote of Sen. Claire McCaskill, a Democrat from Missouri. “We speak out on issues like this. This is advocacy,” she says.

The ad from the League of Women Voters may be tough and a bit out of character, but the 30-second spot is doing what the mainstream media failed to do – draw attention to Brown’s new position.