The voters haven't liked the 113th Congress for a while now -- believing they are more interested in arguing and ideology than in compromising for the good of the country (to fix the economy, create jobs, etc.). And that approval of the job they are doing (or rather -- not doing) is not improving.
The chart above shows the latest polls taken on congressional job approval (all taken in the month of May), and the average of those polls by RealClearPolitics. That average is a miserable 13.4% approval, and no poll rates the approval of Congress higher than 16%.
This should worry the hell out of incumbents in competitive districts. It should also worry the Republicans, who have the majority in the House right now -- because the last few times there was this much discontent among voters, control of the House was flipped between the political parties (and the Democrats just need to win 17 seats to do that).