1. The Six-Year Curse For Presidents
Mid-term elections are usually bad for the president's party, and that holds true for second-term presidents. Since the ratification of White House term limits, five out of the six two-term presidents have lost seats after re-election -- an average of 29 in the House and six in the Senate, according to election analyst Charlie Cook.
In 2006, George W. Bush lost 30 seats in the House and six in the Senate. In 1986, Ronald Reagan lost five in the House and eight in the Senate. In 1966, Lyndon Johnson lost 47 seats in the House and 4 in the Senate. In 1958, Dwight Eisenhower lost 48 in the House and 13 in the Senate. In 1950, Harry Truman lost 29 seats in the House and six in the Senate. (The one exception was 1998, when a strong economy and voter backlash against the Republican-led impeachment of Bill Clinton helped Democrats pick up five seats in the House and break even in the Senate.)
David Mayhew, a political science professor at Yale University, parses the data from 1914 to 2008 and finds "a special six-year itch on the Senate side." That is, second-term presidents typically lose more Senate seats. He attributes it in part to the dynamics of the six-year terms, waning presidential coattails and voter fatigue with the party in the White House.
2. Democratic Voters Don't Turn Out In Mid-Terms
Non-presidential elections tend to be low turnout affairs dominated by older, white, male and conservative voters who prefer Republicans. Over the last three decades, voter turnout has averaged nearly 60 percent in presidential elections and roughly 40 percent in mid-term elections, according to data compiled political scientist Michael McDonald.
What accounts for the difference? "Core Democratic constituencies — youths, minorities and the poor — tend to see their participation decline more precipitously in midterm elections than the Republican core constituency of older affluent whites," explained McDonald, a professor of government and politics at George Mason University.
The most salient factor is age: older voters disproportionately turn out in mid-terms, and they've moved to the Republican Party in recent decades.
"There's a tendency that helps Republicans here and it's mainly because of age," said John Sides, a political scientist at George Washington University, describing age as "the way in which turnout patterns have most clearly worked to the Democrats' disfavor."
The chart below, via the Cook Political Report, illustrates this dynamic.
3. Republicans Have A Mathematical Advantage In The Senate
The million-dollar question is whether Republicans will pick up the six seats (on net) needed to take back the Senate majority.
"Right now, I'd say control of the Senate is a coin-flip," said Sides. "We'll know more as the races evolve, but I'd say it's more likely to tip in Republicans' favor."
The senators who are defending their seats this November were elected in 2008, a wave election in which Democrats picked up eight seats, many of them in conservative states they ordinarily may not have won. As a result, they're defending 21 seats, including in Arkansas, North Carolina, Louisiana and Alaska, all of which lean Republican. The GOP, by contrast, is defending 15 seats -- all but one of them in conservative states where Democrats have a tougher time competing. The lone exception, Maine, features Sen. Susan Collins, a popular incumbent who is widely seen as a safe bet for re-election.
"Senate Republicans need to make up a lot of ground but are battling on favorable territory," noted McDonald.
Sides argued that Obamacare won't be a core issue but could, on balance, work against red state Democrats who helped pass the law but haven't faced voters yet.
Two important factors could cushion Democrats here. The first is incumbency and the name-recognition that comes with it. The second is the GOP's recent tendency to self-destruct on the Senate stage by nominating extreme candidates. Republicans blew their chance of winning the majority in 2010 despite a wave election, and somehow lost two seats in 2012 despite enjoying an advantage in the number and partisan tilt of states where Senate seats were up for grabs. This cycle, the Republican establishment is as determined not to repeat those mistakes as Democrats are to overcome the political winds blowing against them.
4. The House Map Is Skewed Toward The GOP
The House is where the GOP's advantage is strongest. Most analysts believe Democrats don't have a serious chance of winning (on net) the 17 seats needed to take back the majority. In fact, some political scientists project that Republicans will gain seats and slightly expand their already sizable majority of 232 seats to 200 seats.
President Barack Obama's approval rating has declined, like most presidents in their sixth year in office, which doesn't help Democrats. Nor is the gradually improving economy expected to give Democrats the sort of boost they'd need to overcome their disadvantages in the House. On the flip-side, Republicans netted so many House seats over the last two elections that there isn't much left for the taking.
"In the House there's very little chance of a wave this year," said Sides. "I think what you see in the House is mostly a status quo election with very few seats changing hands, and with the Republican majority staying solid."
Political scientists debate the extent to which partisan redistricting following the 2010 elections helped seal the GOP's grip on the House. It certainly didn't hurt. But their more salient advantage is inherent: Democratic voters are disproportionately packed into deeply liberal urban districts, while GOP voters tend to be sprawled across the map and are especially dominant in rural regions that constitute larger chunks of the country.