Nowhere is this dirty business more deftly practiced than in our fair commonwealth. That's why H.B. 2420, a bill to bar gerrymandering that was introduced in Harrisburg this week with 90 cosponsors, is so vital to the fragile health of democracy in Penn's Woods.

If you consider yourself a moderate or independent, this reform should be of particular concern to you. Why? When gerrymandering runs amok, lawmakers have very little motivation to worry about what moderate voters think or want. Moderates' clout simply becomes too weak to cost anybody his job.

You can read up on the bill on the Web site of one of its prime backers, the League of Women Voters (www.palwv.org). Then take up the league's invitation to tell your elected representatives that you want them to kill the gerrymander.

Gerrymandered districts snake across the land in ways that make no geographic sense. Their logic is partisan self-interest. Unfortunately, because H.B. 2420's well-targeted reforms hit lawmakers where they live - their job security - its passage is no sure thing.

Josh Shapiro, a Montgomery County Democrat, is deputy speaker of the House and its point person for reform. He backs H.B. 2420 strongly. Even with his support, though, my sense after talking to legislators is that this bill won't go anywhere without a strong push from the public. And that needs to happen by June, if the reforms are to affect the next bout of line-drawing after the 2010 census.

Help give that push, because gerrymandering does a lot of mischief.

Last week, I wrote about towns getting sliced and diced among three, even four election districts, meaning they're not really represented at all. I noted how incumbents nestled into a safe district tend to care less about their constituents than they do about the far-flung interests that hand out campaign cash.

Today, let's zero in on a subtler evil: how gerrymandering muffles the vast political center, while giving undue influence to the screamers on the margins, the one-issue activists and the partisan ideologues.

How is that? Let Jim Leach, a former congressman and current national chair of Common Cause, explain:

"When you hold a gerrymandered seat," Leach said, "you don't have to worry about being really challenged in the fall. The other party doesn't have enough votes. The only way you can lose your seat is a primary challenge from within your own party. Those tend to come from the extremes."

In other words, the only way such incumbents can put their seats at risk is to annoy the extreme partisans in their own party. And what annoys them? Exactly the same things that moderate voters tend to like: compromise, bipartisanship, solutions that borrow from each side's best ideas, that defuse emotional wedge issues.

By Leach's arithmetic, this syndrome produces a politics where candidates get chosen, elected and disciplined by a tiny, partisan fraction of the electorate: "The country is divided roughly a third, a third and a third: Democrat, Republican and independent. The independents get no say in picking the candidates. In each party, its more ideological half tends to control who runs and wins in primaries. So you're down to one-sixth. In the end, only about one out of four people bothers to vote in primaries. So 1/24th of the electorate calls the shots."

We've seen the effects this has on how incumbents behave: zero taste for compromise, a zest for harsh words and wedge-issue votes, gridlock on major issues.

Leach is a Republican: "In a free-enterprise society, competition is normally considered a plus, right? All gerrymandering does is weaken competition. An uncompetitive democracy can't be a vibrant democracy."

Leach thinks H.B. 2420 hits most of the right notes; he knows what a good redistricting process looks like, because his home state of Iowa has the one most reformers regard as the best. And this is how much he believes in doing things the right way:

When Iowa redrew congressional districts after the last census, it wiped out Leach's district and prompted him to retire. Is he bitter?

"Not at all. The process was absolutely fair. Yes, I was personally disadvantaged, but I support the result, because it was reached fairly."

We need some Jim Leaches on the banks of the Susquehanna. Maybe your letters, e-mails and phone calls about H.B. 2420 can summon some to the fore.

To comment, call 215-854-4243 or e-mail csatullo@phillynews.com.