Rob Richie is executive director of FairVote, a nonpartisan organization based in Takoma Park.

Barack Obama won a clear victory in the presidential election, and Democrats won 25 of 33 Senate races. But they failed miserably to win the House, falling short by more than 30 seats. How could the same voters deliver such different outcomes?

The answer lies in the way we elect House members. Because of where each party’s voters live, Republicans have an intrinsic advantage under today’s winner-take-all voting rules. Left unreformed, the rules will distort House representation for at least a decade.

Last week, Democrats won the most votes in contested House races. FairVote’s post-election analysis of partisan voting trends suggests an underlying national preference toward Democrats of 52 percent to 48 percent. (If there had been no incumbents and each party had run a candidate in every district, the Democrats would have won 52 percent of the votes.) With a comparable edge in 2010, Republicans gained 64 seats to take the House. But this year, winning a House majority would have required a much bigger swing toward Democrats, as much as 55 percent of the vote, a historical high.

Incumbency and campaign spending helped Republicans, but their biggest advantage is structural. We elect House members in 435 districts, each with one representative. If ordered by their partisan leanings, 241 districts tilt toward Republicans. Only 194 lean toward Democrats. Of districts where one party’s underlying advantage is at least 8 percentage points, Republicans have the edge in 195 and Democrats in 166.

This partisan tilt has dramatic consequences. Partisanship is the dominant factor in determining election outcomes. The ­major-party presidential candidates raised more than $2 billion, but they focused on just 10 states — all of which were swing states in 2008. The remaining states’ electoral votes were accepted as locked up for one party or the other because of their underlying partisan tilt.

House races can be similarly grouped. In 2010, Democratic incumbents went 139-0 in the most Democratic districts but were swamped in Republican districts. This year, Democrats won 176 of the 177 most Democratic districts, but Republicans didn’t need to win a single Democratic-leaning seat to keep their majority.

Independent redistricting alone will only modestly reduce this partisan bias. Although gerrymandered maps boosted Republicans in several states, maps drawn by independent commissions would still leave Democrats at a disadvantage because of their relative concentration in urban areas. More than two-thirds of the 68 most one-sided districts are Democratic. But Republicans dominate in 192 of the 323 remaining districts where one party has an edge of at least 6 percentage points. Last week, Democrats gained only three seats in those 192 districts.

Democrats have faced this bias for decades — in 1997, the GOP had an edge of 43 House seats in districts largely drawn by Democrats — but they could win House majorities because their “blue dog” centrists could outperform their presidential nominees. As ticket-splitting has declined, those candidates are losing — and the longtime partisan bias in districts is exposed.

The good news is that bias can be removed through statutes replacing single-member district elections with fair voting plans that are grounded in our own traditions. As recently as 1968, many House members were elected in larger districts that had more than one representative. Fair voting alternatives to winner-take-all elections are increasingly common in local elections.

FairVote has drawn voting plans for every state that are designed to allow like-minded voters to elect representatives in proportion to their voting strength. Candidates would run in larger congressional districts, with three to five seats. Rather than nine single-member districts, for example, Massachusetts would have three districts that would each elect three representatives. 

The simplest fair-voting system would give each voter one potent vote. In a three-seat district, a third of voters would have the power to elect a preferred candidate. This math means that like-minded voters reflecting a district’s left, right and center would consistently elect preferred candidates. In Massachusetts, each district would probably elect a Republican, a Democrat and an independent-minded centrist leaning to the left. Louisiana’s middle seat would likely go to a more conservative centrist.

Nearly every voter in every election would end up with shared representation — meaning both Republicans and Democrats winning in every corner of every state. Nationally, partisan bias would be eliminated, so the party clearly having a better year would almost always win more seats.

Fair voting has great political potential, grounded in major parties that see the value in their candidates winning seats across the nation. It also would probably boost the election of women and racial minorities.

Simply highlighting existing biases helps focus attention on Congress. House Republicans kept their majority this year only through incumbency and structural bias. We should ask for fairness — and openness to reforms that establish what every American can support: a level playing field.