Congressional Democrats hold a clear advantage in the last Pew Research poll. If the congressional election was held today 49 percent of Americans would plan to vote for the Democrat in the district while only 43 percent would plan to vote for the Republican. That is a significant six point lead.
Thanks to natural partisan sorting and gerrymandering Republicans currently hold a huge edge in the House. That is how they were able to win control last year even though more people voted for Democrats. To overcome this big Republican advantage Democrats would need to not just win the 2014 election but dominate it. A six point victory would probably be a large enough win to take back the House and keep the Senate.
Obviously, a poll roughly a year out from the election is not the best predictor. Things will change and new development will arise. The Republican party has a lot of time to improve their imagine and that is what any sane political party would try to do. Of course if the Republican party doesn’t make a real effort to fix their current problems they are on track to a potential lose.