Why Senate Democrats eliminated filibuster nominations, in one graph

For the last year, during repeated fights over nominations, the question of whether this confrontation or that confrontation would be the trigger for doing away with filibusters has hovered over the Senate.

The exact timing of when the change would come remained in doubt until the moment Majority Leader Harry Reid (D-Nev.) made his move Thursday. But while the timing was in doubt, the end result wasn’t: Long-term shifts in American politics toward ever-greater polarization have made the end of filibusters all but inevitable.

The chart above, based on the work of political scientists Keith T. Poole of the University of Georgia and Howard Rosenthal of NYU, shows how both houses of Congress have changed. From the late 19th century through the first few years of the 20th, Congress was sharply polarized between the two parties. But from the 1920s through the 1970s, party lines blurred, in part because of the large number of Southern conservatives in the otherwise liberal Democratic Party.

PHOTOS: 2013’s memorable political moments

Those Southern, white conservative Democrats shaped the Senate of the 20th century into a place that elaborately protected the rights of a legislative minority -- themselves. They used the minority’s power to block civil rights measures for decades. Over time, other groups in the Senate picked up the same strategy, using filibusters to block legislation or nominations they opposed.

But the Senate’s elaborate protections of minority rights survived in large part because party discipline in the Senate, indeed in Congress as a whole, was weak during that period. The idea that a majority should be able to work its will was hard to enforce when the majority was a frequently shifting, bipartisan coalition that changed from issue to issue.

In more recent decades, as Southern whites shifted out of the Democratic party and northern social moderates deserted the GOP, the two parties have once again reached 19th century levels of polarization. The most liberal Republicans in Congress are several steps to the right of the most conservative Democrats. As a result, Congress has moved toward a parliamentary system characterized by near-lock-step party voting and deep, consistent ideological differences between the two parties.

Bipartisan coalitions have dwindled, and so have the incentives on the other side of the aisle to bend the majority party’s agenda to accommodate the minority. For both parties, winning a majority means pressure from constituents to actually adopt the majority’s program.

PHOTOS: The battle over Obamacare

Whether these developments are good, because they mean voters can hold the majority accountable for its policies, or bad, because they increase divisiveness, can be the subject of lively debate. But what’s hard to deny is the shift has caused the filibuster to appear more and more as an anachronism, impossible to keep in an era defined by the ethos of “majority rules.�

Sooner or later, in these polarized times, the filibuster was doomed. Reid’s actions Thursday merely determined the timing.

Follow Politics Now on Twitter and Facebook

[email protected]

Twitter: @davidlauter