In a D+4 election, the Democrats win the House. In 2010 the Republicans established many districts with R+2, R+3, and R+4 advantages and very few with D+1 top D+5 advantages. Democrats got very sure seats. The slightly-R seats would have been tailor-made for moderate Republicans, but the Republicans elected pols better suited for R+30 districts.
Gerrymandering obviously can't help Republicans in the Senate.
Cook and various other analysts have suggested it's basically going to take D+7 or D+8 to take back the House, due to the large majority and rigged districts. Of course, seeing as it is by district, it could require less depending on how all 435 races play out, but meh. How were you going by D+4? Just curious.
On that note, the generic congressional poll has been favorable to Democrats for the past 12 months now. We could actually have a good shot at a D+4 House PV this year, if not higher, depending on what polls you trust. That doesn't even really fully account for the effects of straight ticket voting, either.
