The Incredible Shrinking Senate Majority

As voters' behavior has changed, control of the upper chamber has become increasingly precarious.

While both parties are pouring vast sums into the battle for Senate control, the best advice for whichever side wins the majority in November may be three simple words: Don't unpack everything.

Structural changes in voters' behavior are making it tougher for either party to amass, much less sustain, a comfortable Senate majority — except in rare circumstances. "At this point, the natural division of party strength in the Senate is very close to 50-50," says Alan Abramowitz, an Emory University political scientist. "I think we are going to be seeing small majorities for some time." Small majorities increase the chances that control of the chamber will flip between the parties more frequently. And that prospect increases the odds that the Senate will see more polarization, less accomplishment — and even more campaign spending.

The big dynamic pointing toward precarious Senate majorities is the increasingly parliamentary nature of congressional elections. In the first decades after World War II, many voters routinely split their tickets, supporting one party's presidential nominee and candidates from the other party in House and Senate races.

But after peaking in the 1970s and 1980s, such split-ticket balloting has steadily declined in our polarized political era. Senators are better known than House members, but attitudes toward the president are increasingly buffeting both groups. In the 2006 midterm election, Republicans won six of the 10 Senate races in states where exit polls showed President Bush's approval rating reaching 46 percent or above — but lost 19 of the 20 in states where he stood at 45 percent or below. In 2010, Democrats won Senate races in nine of the 10 states in which President Obama's approval reached 48 percent or higher — and lost 13 of the 15 states that gave him lower marks.

Rather than judging Senate candidates entirely as individuals, more voters appear to be assessing them as part of a team — and voting on whether they want that team to steer the Senate. This tendency isn't insurmountable. In 2012, Democrats won Senate seats in Indiana, Missouri, Montana, and North Dakota, even as those states rejected Obama — largely because their candidates benefited from flawed Republican opponents. But the long-term trend is for each side to dominate the Senate seats in the states its presidential candidates usually win. With each party holding a presidential edge in just under half the states, and few true swing states remaining, that promises a tenuously balanced Senate.

Consider the current Senate map. Twenty-six states voted for Obama both times; Democrats control 43 of their 52 Senate seats. Twenty-two states rejected him both times; Republicans hold 34 of their 44 seats. (The remaining states, Indiana and North Carolina, backed Obama in 2008 and flipped against him in 2012; the two parties split their four Senate seats.) Not all those seats are secure, but the overall pattern is strong: Almost four-fifths of each party's senators represent states that supported their party's presidential nominee in both 2008 and 2012.

This sorting intensifies the Senate's polarization. Legislators sent by conflicted constituencies, such as the old Southern Democrats or Northeastern Republicans, have often been the Senate's deal-makers; they have a natural incentive to seek bipartisan compromises. Those from states that lean solidly to one side have more incentive to reliably line up behind — or against — the president. Exceptions persist on both sides of that alignment. But as the chamber's sorting continues, the overall effect is "that the Senate [will] be a worse and worse place to get anything done," worries veteran Democratic consultant Jim Jordan.

November's election might produce more cross-pressured legislators open to compromise. With Obama's approval sagging, GOP candidates are seriously competing in Democratic-leaning Colorado, Iowa, Michigan, and New Hampshire. Democrats are building plausible, if uphill, challenges in red-leaning Georgia and Kentucky.

But it's more likely that this fall's election will accelerate the Senate re-sort. The GOP's best takeaway chances are the six Democratic-held Senate seats in states that twice rejected Obama, plus North Carolina. The four Democratic incumbents seeking reelection in those states (the other three seats are open) are showing surprising resilience in polls. But each faces a formidable undertow of local alienation from Obama. Each GOP win would replace a relatively moderate Democrat with a more conservative Republican possessing less incentive to compromise.

Democrats nervous about November can calm themselves by anticipating 2016. In that presidential year, the tightening link between Senate and White House preferences will favor them. After benefiting from the 2010 tea-party wave, Republicans must defend Senate seats in seven states that backed Obama twice. Even if Democrats lose the chamber this fall, they could recapture it in 2016.

Yet the dial will turn again in 2018, when Democrats must defend the red-state seats they won in 2012 against flawed GOP nominees. By now the larger picture should be clear: Neither side is positioned to establish a lasting Senate advantage. In a more rational political environment, fragile Senate majorities would encourage more negotiation between parties that recognize neither holds a dominant hand. Instead, instability is compounding rigidity as the Senate's partisan divide hardens.