The Red and Blue Senate

Many posts have discussed polarization in Congress. Ronald Brownstein writes at National Journal:

The tightening correlation between presidential and Senate voting represents a back-to-the-future trend in national politics. Through the first half of the 20th century, party-line voting was common. After Franklin Roosevelt's first two victories in 1932 and 1936, for instance, Democrats held 89 percent of the Senate seats in the 40 states that supported him both times.

This relationship frayed later in the century, as more voters split their ticket between presidential and Senate races. That was especially true in the South (and to some extent in the Mountain West), where many voters who had shifted toward GOP presidential candidates still supported Democrats in Senate and House races. The result was that the GOP controlled only about half the Senate seats in the states that twice voted for Richard Nixon (in 1968 and 1972) and Ronald Reagan (1980 and 1984).

The large number of senators elected from states that leaned toward the other party in presidential elections encouraged the Senate's culture of compromise and negotiation from roughly the 1950s through the 1980s. Senators who'd been elected, in effect, from behind enemy lines were natural dealmakers: Representing voters with mixed loyalties, they had a clear self-interest in suppressing partisan conflict.

But those instinctive bridge-builders are vanishing. Since the 1980s, party-line voting between presidential and congressional races has steadily increased. The share of Senate seats controlled by the president's party in the states he carried twice rose to two-thirds after Bill Clinton's reelection and to three-fourths under George W. Bush. Following Obama's two victories, Democrats now hold 83 percent (43 of the 52 seats) in the 26 states he carried twice.