Why the Senate Will Only Get More Polarized

It's too early to confidently predict which party will hold the U.S. Senate after November's election. But it's a safe bet the next Senate will more closely reflect the nation's entrenched red-blue presidential divide. And that's a recipe for even more polarization and gridlock.

This year's races will likely provide more evidence of voters' growing inclination to support Senate candidates from the same party as that of their presidential choice. This has made it much tougher than a generation ago for either party to elect senators from states that typically back the other side in presidential elections.

The tightening correlation between presidential and Senate voting represents a back-to-the-future trend in national politics. From the early through mid-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 20th 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.

Republicans this fall might swipe a Senate seat in one or two of those blue-tilting states (Michigan is their best opportunity), but the bigger change is looming in red places. Boosted by the GOP's weakness late in Bush's presidency, Democrats hold 10 of the 44 seats in the 22 red-leaning states that twice opposed Obama. This fall Democrats must defend six of those seats: reelections for incumbents in Alaska, Louisiana, and Arkansas, plus open seats in West Virginia, South Dakota, and Montana. In addition Democrat Kay Hagan is seeking reelection in North Carolina, one of the two states (along with Indiana) that flipped from Obama to the GOP in 2012.

Obama's local weakness threatens all of these Democrats. In both the 2006 and 2010 midterm elections, exit polls found that Senate candidates from the president's party lost almost every race in states where his approval rating fell to 47 percent or below. Gallup's most recent polling put the president's approval rating at 43 percent.

The 2012 Democratic Senate victories in solidly red Missouri, Indiana and North Dakota show it's possible to buck these trends: each of those Democratic candidates survived an anti-Obama tide by exposing vulnerabilities in flawed Republican opponents. Yet even the most skilled politicians on both sides are finding it tougher to surmount voters' growing tendency to vote more for the party and less for the individual.

This growing connection between presidential and Senate voting has implications that extend well beyond the 2014 battle for control. Red-state Democrats like Louisiana's Mary Landrieu and West Virginia's Joe Manchin, and blue-state Republicans like Pennsylvania's Pat Toomey and Maine's Susan Collins, are often the most eager Senate dealmakers. But the trend toward party-line voting means there will be fewer of them. With each party holding the presidential advantage in about half the states, it also means "the contemporary Senate will be very unlikely to see big majorities" for either side, and will likely experience more frequent switches in control, notes Emory University political scientist Alan Abramowitz.

Fewer dealmakers mean fewer deals; and narrow, tenuous majorities will encourage the minority party to wage war to deny the majority any accomplishments that could reinforce their fragile advantage. Both of those trends promise yet more Senate polarization. "It changes the internal dynamics," notes Abramowitz. A Senate that more precisely tracks the red-blue divide is less likely to transcend it with creative compromise.

In an era of unstable majorities, whichever side controls the Senate majority after November will likely find its advantage fleeting. But these underlying changes are transforming the Senate more lastingly -- and not for the better.