According to Gallup:
Presidents who retain majority job approval from Americans at the time of midterm elections are much less likely to see their party suffer heavy seat losses than are those with sub-50% approval ratings. Since 1946, when presidents are above 50% approval, their party loses an average of 14 seats in the U.S. House in the midterm elections, compared with an average loss of 36 seats when presidents are below that mark. … The clear implication is that the Democrats are vulnerable to losing a significant number of House seats this fall with Barack Obama’s approval rating averaging 45% during the last two full weeks of Gallup Daily tracking. The Republicans would need to gain 40 House seats to retake majority control.
It’s worth bearing in mind that many other political metrics — from voter intensity to the generic congressional vote to Obama’s massive loss of support among Independents to GOP dominance on the issues — are more problematic than the president’s approval rating. But as Gallup points out, that’s problematic enough.