We understand her motivations, but it’s still a cause for dismay that House Minority Leader Nancy Pelosi (Calif.) has decided to use threats to keep members of the Democratic Caucus in line on key votes. It’s one more step in the rampant partisan polarization of Congress — and all of American politics — that’s doing nothing to solve the nation’s problems.

As Roll Call first reported Monday, Pelosi is pushing a change in Caucus rules — one almost certain to pass even after being postponed — giving the Steering and Policy Committee the authority to approve ranking Democrats (i.e. would-be chairmen) of subcommittees on the three most powerful House panels, Appropriations, Energy and Commerce, and Ways and Means.

Democratic aides made no bones about the purpose: to discourage Democrats on those committees from voting against the party line out of fear that they could be passed over for the subcommittee leadership posts they covet. It’s a response, they said, to the fact that 16 Democrats broke with leadership in December to support the GOP’s Medicare prescription drug bill. At the time, Pelosi said there would “be no passes” for Democrats who voted for the bill. This is her method of ensuring it.

Let’s be clear: This is the Democrats’ response to what has already happened on the Republican side, where leadership exerts iron discipline on Members and has often used waivers, committee reorganizations and straight pass-overs to deprive wayward (read “moderate”) Republicans of hoped-for posts. The message isn’t lost on GOP Members, and Pelosi is now playing catch-up.

But it’s all part of a lamentable process reminiscent of a gang war. Bipartisanship is long dead in the House. Comity scarcely exists. House Republicans treat Democrats with even less respect than Democrats accorded them when they were in the majority, which wasn’t much. So, Pelosi is trying to consolidate her forces and impose discipline — she’s “going to the mattresses,” as it were.

But look at the upshot: The Democrats most likely to depart from party orthodoxy are those in contested districts. Making them toe the line on difficult votes could easily ensure their defeat. That seems self-defeating for Democrats if they ever want to regain the majority. But what’s worse is that it exacerbates polarization. Gerrymandering has ensured that there are fewer and fewer contested seats with each passing election cycle. Democrats, ensconced in safe seats, are trending more liberal. Republicans, similarly safe, are becoming more conservative. “Reaching out,” once a respected tradition in Washington, has almost become a synonym for treason. America’s problems don’t get solved when its leaders are constantly at war and the electorate, deprived of the power to change things, just gets turned off.