Dem disadvantages: Obama, turnout and 172 years of history

As the votes will soon trickle in tonight from congressional races across the nation, it's important to remember that the so-called "6-year itch" is especially bad news for sitting presidents.
President Obama's approval ratings are dragging down House and Senate Democrats, and it appears that the Republicans have met or exceeded the Dems' ground game tactics that produce strong voter turnout. 
Nonetheless, midterm elections are rarely good news for the White House, particularly in a second term when voter fatigue with the man in the Oval Office is peaking. At a time when public approval of Congress in 2014 is at record lows, dissatisfaction with the president trumps all other dynamics.

The Pew Research Center recently offered this perspective: 
"... If 2014 follows the (historical) trend, Democrats are almost certain to lose seats in the House and Senate this November, and many pollsters predict as much. As Knight notes, since 1842 the President’s party has lost seats in 40 of 43 midterms — the exceptions being 1934, 1998 and 2002."

The Knight referred to in that quote is a reference to researcher Brian Knight of Brown University.
Here's more from Pew:

"A recent paper by Brown University researcher Brian Knight seeks to evaluate (a) surge-and-decline theory, as well as two competing explanations of why the president’s party nearly always loses seats at the midterms: a “presidential penalty,”  or general preference among midterm voters for expressing dissatisfaction with the president’s performance or ensuring that his party doesn’t control all the levers of government, and recurring shifts in voter ideology between presidential and midterm elections. Knight concluded that while all three factors contribute to what he calls the “midterm gap,” the presidential penalty has the most impact.

As for midterm turnout, there is this:

"Though political scientists long have noted the midterm dropoff, they don’t agree on precisely what it means. In an influential 1987 article, James E. Campbell theorized that “the surge of interest and information in presidential elections” typically works to the advantage of one party or the other; that party’s partisans become more likely to vote, while those of the disadvantaged party are more likely to stay home during presidential elections. Independents, “lacking a standing partisan commitment…should divide disproportionately in favor of the advantaged party.” Midterm elections lack that “wow” factor, according to Campbell, and turnout among both partisans and independents return to more normal levels and patterns."
Pew has also focused on House races, which have received little attention even as the GOP is expected to add seats.
The research center has found an amazing disconnect between voter turnout in midterm House elections and the competitiveness of a particular seat:

"... Our analysis shows little, if any, correlation between a House election’s competitiveness (measured by the winner’s victory margin) and turnout.

"In the Tea Party year of 2010, for example, overall turnout in House races was 40.7% of estimated eligible voters. That year, the nation’s highest turnout was in Wisconsin’s 5th District, where 62.4% of estimated eligible voters cast ballots and Rep. F. James Sensenbrenner cruised to a 42-percentage-point victory. The nation’s tightest race in 2010 was in Illinois’ 8th District, where Republican Joe Walsh edged Democrat Melissa Bean by just 290 votes; 41.7% of estimated eligible voters cast ballots in that contest. (The lowest turnout rate in 2010, 19.5%, came in Texas’ 16th District, which Democrat Silvestre Reyes won by a comfortable 21.5 percentage points.)"