Senate and House results increasingly track those presidential preferences. After peaking in the decades around World War II, the share of voters who split their tickets by backing different parties in presidential and congressional races has steadily declined since the 1970s.
This shift, as I've noted, has made it tougher for the parties to win Senate or House seats that are, in effect, behind enemy lines—in places that usually prefer the other side's presidential candidates. In the House, just 17 Republicans and nine Democrats represent districts that voted for the other party's presidential nominee in 2012. After the 1996 election, 110 House members represented such split-ticket districts.
In the Senate, Democrats hold 82 percent of the seats in the 26 states that backed Barack Obama each time (43 of 52). That's the highest percentage of Senate seats any two-term president's party has held in the states that twice backed him since the 1930s, when Democrats held 89 percent of the seats in states that Franklin D. Roosevelt won in 1932 and 1936. Meanwhile, Republicans hold 77 percent of the Senate seats in the 22 states (34 of 44) that rejected Obama both times.
In one respect, the election is guaranteed to deepen this divide. When presidents stumble, as Obama has since 2012, their parties slip everywhere. That decline threatens them the most in the places that were always resistant to them.
When presidents stumble, as Obama has since 2012, their parties slip everywhere.
In 2006, George W. Bush's second-term weakness decimated House and Senate Republicans in blue-leaning areas. The same fate seems likely this year for Democrats uneasily surviving on red terrain. With Obama's approval rating among white voters in these states sometimes falling below 30 percent, Democrats could lose all six of the Senate seats they are defending in states that twice voted against Obama—Alaska, Arkansas, Louisiana, Montana, South Dakota, and West Virginia. In North Carolina, which backed Obama in 2008 before preferring Mitt Romney in 2012, Democratic Sen. Kay Hagan maintains a nail-biter of a lead.
These reversals could be mitigated by Republican losses in Georgia and Kansas (or, less likely, Kentucky), resulting in the turnover of red-state Senate seats now held by the GOP. But it's virtually certain that after November, Republicans will hold more than 80 percent—and maybe 90 percent—of Senate seats in the anti-Obama states. In the House, the numbers could be even more lopsided: Republicans may take most of the nine seats Democrats hold in districts that favored Romney.
Obama's decline is also allowing Republicans to mount challenges in blue-leaning places—just as Bush's troubles enabled significant Democratic red-state gains in 2006. In the House, Republicans are seriously contesting about two dozen of the 36 Democratic-held districts where Obama won 55 percent of the vote or less. (About half a dozen of the GOP's incumbents in Obama districts are also threatened.) The blue-to-red Senate opportunities for Republicans are scarcer. In states that twice supported Obama, Democrats lead comfortably in Michigan, and narrowly but stably in New Hampshire. Republicans are better positioned in Colorado and Iowa—but must overcome potent Democratic turnout organizations.