Looking at historical precedents, Harry Enten finds that Democrats have little reason for optimism in 2014:
The only year in which the president’s party improved its position by more than two percentage points was 1958, when Republicans under President Dwight D. Eisenhower picked up nearly six points on the generic ballot. The party holding the White House gained two points or less in four other years (1970, 1974, 1982 and 1994). Every other year saw at least a small decay in the presidential party’s position on the generic ballot. The median year was 2010, when the Democrats’ position declined 5.3 percentage points compared to the Republicans’. In the past five elections, the median drop was similar — -5.7 points.
Bernstein adds his analysis:
Is there any good news for Democrats here?
Not much. First, most of the large poll movements in Enten’s chart appear to be regression to the mean. Three of the four instances in which polls changed more than 10 points — all negatively for the president’s party — came after the president’s party held a 20-point lead or more in the generic ballot early in the presidential year. So perhaps a major decline for Democrats is unlikely. Unfortunately for Democrats, however, the one significant movement in favor of the president’s party was probably a result of exactly the same phenomenon. In 1958, Republicans improved from an awful 20-point deficit to a less awful 14-point gap. They received a shellacking in November, anyway. With the generic ballot measure essentially even now, there is no precedent for a sizable rally by the president’s party under similar circumstances.