Midterm elections rarely go in the president's favor. In fact only three midterms in the past century have seen the president's party gain seats.
But President Obama has had it especially tough. He's already lost more House seats than any president since Eisenhower:
Obama has lost 63 House seats since he took office, taking into account the Democrats' small gain in 2012. The Democrats are expected to lose somewhere between 5 to 12 seats or more this year, in which case Obama would comfortably overtake Eisenhower. But if the loss reaches around 20, then Obama will have lost more seats than any president since FDR, who lost 108 across his three midterms.
Further Reading: Ezra Klein explains how Republicans are heading for a landslide victory despite being less popular.