Sabato’s Crystal Ball observes that the president’s party tends to suffer in down-ticket races. This applies not just to Congress but also to state government:
Since World War II, the president’s party has gained a net number of state legislative seats in just one of 17 midterms: George W. Bush’s 2002 midterm. (This election was a rare positive for the president’s party across the board — the exception that proves the rule, given the unique impact of Sept. 11.) The loss of power by the president’s party can be particularly damaging in election years at the start of the decade. For instance, Republicans had total control of 25 out of 50 state legislatures after the 2010 elections, which was their highest mark since 1952. …
This is not to suggest that the presidential trophy is some sort of cursed, booby prize, like wearing Sauron’s “One Ring” in Lord of the Rings or being named the drummer of Spinal Tap. The American presidency is immensely powerful, and the person who holds it has wide latitude to craft national policy; ladle out immense portions of patronage and powerful lifetime appointments; and, effectively, start and end wars abroad. But winning the presidency has its downsides, too — if not directly for the person who holds the office, then certainly for his or her party.
The Dish 