Notwithstanding the four Democratic red-state triumphs, this election mostly confirmed the rise since the 1970s of parliamentary-style party-line voting between presidential and Senate races. With only three exceptions (all for Republican candidates), at least 80 percent of Obama and Romney voters also backed their party's Senate candidate in each of the 14 top races for which exit polls are available.
Within that broad overall trajectory, though, Democrats benefited from extraordinary levels of unity. All but three of those 14 Democratic Senate candidates won support from at least 92 percent of Obama voters. That's well beyond even the level of party unity evident in 2004 when George W. Bush won reelection.
Republicans this year consistently suffered slightly greater defections. The Democratic Senate candidate inspired more ticket-splitting among Romney voters than the Republican did among Obama voters in every race except four: Connecticut and Massachusetts (where Democratic candidates Chris Murphy and Elizabeth Warren still won handily behind Obama's comfortable margins) and Nevada, where Democratic nominee Shelley Berkley lost amid ethics questions. (In Wisconsin, Democrat Tammy Baldwin and Republican Tommy Thompson inspired equal crossover, but that allowed her to win because of Obama's victory in the state.)
This imbalance was particularly pronounced in the Democratic red-state victories. In Missouri, reelected Democratic Sen. Claire McCaskill won 20 percent of Romney voters but lost only 1 percent of Obama supporters. In Democrat Joe Donnelly's winning Indiana campaign, he captured 15 percent of Romney voters but surrendered only 3 percent of Obama's.
Unique factors, of course, critically contributed to the Democratic red-state gains: Sen. Jon Tester in Montana and Heidi Heitkamp in North Dakota beat House incumbents with prickly personal images, and Donnelly and McCaskill each bested opponents who immolated themselves with comments about rape. But overall, the Senate results reinforce the sense that an unusually unified coalition fueled the Democrats' gains at all levels.
In November, the older and blue-collar whites that often anchor the conservative end of the Democratic vote tilted even more than usual toward the GOP. As a result, Democrats won behind a remaining coalition of minorities, young people, and upscale white women that not only voted a mostly straight ticket but also expressed strikingly consistent policy views. In the exit poll, four-fifths of Obama voters said that Washington should raise taxes on the affluent; an equal number said that Washington should provide illegal immigrants a pathway to citizenship.
With that consensus in their voting coalition, Senate Democratic aides believe it will be easier to unify their members on both of those central issues. Apart from Donnelly (and to a lesser extent reelected Sen. Bill Nelson of Florida), Democratic Senate contenders didn't make firm commitments to extending the Bush tax cuts for the rich. Some red-state Democrats may still resist immigration reform that offers legalization, but given the strong Hispanic impact on the election's results "we are at a point where Democrats are no longer on the defensive [about it] even in the reddest of states," said one senior Senate Democratic aide.