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.