(As a sequel to our post about U.S. House elections, Sheahan Virgin and I took to the U.S. Senate.)

Despite having one of the Senate's more conservative voting records, Dick Lugar (R-IN) has not been a partisan ideologue. His recent landslide primary defeat to Tea Party-backed challenger Richard Mourdock is the latest sign that reaching across the aisle to build bipartisan policy has become a recipe for a shortened political career.

After a campaign in which Mourdock charged that Lugar was "President Obama's favorite Republican," Lugar defended "constructive compromise" and warned that "unrelenting" partisanship would paralyze American government. His remarks mirrored those of Olympia Snowe (R-ME) when she announced her retirement this spring. Snowe lamented that her approach to governance no longer fit an institution in which partisans "block the other side" and demand reflexive fealty to party.

Snowe called on Americans to see "strength in compromise, courage in conciliation, and honor in consensus-building." But moderates are far more apt to be pilloried and purged than prized and protected. Lugar and Snowe are only the latest in a string of victims of a "do-not-compromise" stance that has been reinforced within both major parties by every new hardliner victory. Ben Nelson (D-NE), Joe Lieberman (D/I-CT), George Voinovich (R-OH), Blanche Lincoln (D-AR), Lincoln Chafee (R/I-RI), Arlen Specter (R/D-PA), and Bob Bennett (R-UT) are among high-profile senators whose history of bipartisanship led to retirement or lost reelection bids. The ranks of moderates in the House have been similarly depleted.

The decline of the middle in the Senate is especially significant given that its rules make one-party rule almost impossible. Because the Senate needs 60 votes to break a filibuster, moderates can assist the majority by voting for cloture or side against the majority until it tempers its proposals. They often inject restraint into a dialogue often soured by venomous rhetoric.

Yet as the divide between the parties has widened and solidified, moderates today face pressure to conform or retire. Those remaining face a dual threat: stronger challenges to holding onto their party's nomination and uphill general elections in the "opposition territory" they disproportionately represent. It's no accident that 2012's most vulnerable Senate incumbents -- Scott Brown (R-MA), Jon Tester (D-MT), and Claire McCaskill (D-MO) -- all represent states that their presidential nominee will likely lose. Overcoming that partisan challenge has become harder in every passing election.

Sen. Snowe has rightly faulted the "corrosive trend of winner-take-all politics," in which winning parties fail to forge bipartisan solutions. But we must go further to understand this behavior. Winner-take-all politics is grounded in winner-take-all voting, in which a plurality of votes earns 100% of representation. We have had those rules for a long time, but not with modern consultants armed with new polling methods, modern technologies, and near-limitless funds. In mastering how to win the game, we have destroyed its ability to produce effective representative government.

While not as enthralling as the battle for power and clash of values that dominate political coverage, election rules matter -- they condition ballot tabulation, voter behavior, and campaign strategy. Winner-take-all influences the composition of government -- who's in it and who's not, as well as how they get there. If Americans are dissatisfied with the latter, they must examine the former.

Winner-take-all rules today simply cannot represent fairly the left, right, and center. In a two-party system, the gain of one side is undeniably the other's loss when third parties and independents are dismissed as mere "spoilers" rather than viable alternatives. Vilifying one's opponent leaves voters with only one viable electoral option: oneself. The apocalyptic rhetoric and black-and-white thinking that pass for political strategy work under winner-take-all.

These tools of negative rhetoric and zero-sum campaigning translate into the habits of governing. With only two viable choices, parties are rewarded electorally for obstruction more than compromise. In turn, partisans seek representatives who will fight more than seek consensus.

Winner-take-all voting incentivizes partisanship, compels centrists to squeeze into restrictive ideological boxes and rewards the "us-versus-them" mentality moderates resist. But it is not part of our Constitution, and our cities and states are already providing a roadmap for change. In Illinois, cumulative voting in three-seat state legislative districts led to shared representation across the state for both parties until 1980, when the legislature was reduced in size and went to one-seat districts. In Minneapolis and San Francisco, the instant runoff form of ranked choice voting rewards candidates who can earn the second-choice support of other candidates' supporters.

Electing representatives interested in compromise and independent thinking in proportion to the share of voters who support them will require structural election change. Pleading with voters to support centrist politicians is not sufficient when the institutional framework of American elections and government discourages and penalizes such behavior. Rather than just criticize Congress as broken, let's act to fix it.