Since the start of the modern two-party system in the mid-19th century, the party of an incumbent president has never captured control of the House from the other party in a midterm election. While many presidents have held the House for their party, in 35 of 38 midterms since the Civil War the incumbent’s party has lost ground.

One reason is turnout. Since 1866, the average turnout rate in presidential elections has been 63%, while it has been 48% at midterm. The drop-off comes disproportionately from the presidential party. Voters from the out-of-power White House party are usually energized—read: angry and eager—to vote against the president, especially by the six-year mark.

Events can encourage voter frustration. Midterm setbacks may be spurred by bad economies, which partly explain losses suffered by the parties of FDR (1938); Harry Truman (1946); Dwight Eisenhower (1958); Ronald Reagan (1982); and Mr. Obama (2010). War fatigue has contributed to other midterm disappointments: LBJ’s Vietnam (1966) and Mr. Bush’s Iraq (2006). …

But don’t those polls showing the deep unpopularity of House Republicans mean that Democrats can make up the necessary ground? Not really. Partisan redistricting and an inefficient concentration of the Democratic electorate generally favors the Republicans. Democrats won nearly 1.4 million more House votes than Republicans in 2010 and still lost the House.