It doesn't matter that John Boehner is as unpopular as Nancy Pelosi on the generic ballot or whatever.
A. Pelosi's unpopularity all came from one side of the aisle, unifying an opposition and allowing them to sell that unpopularity to independents. There is no similar marketing campaign to turn independents against Boehner.
B. If Democrats are somehow able to make Boehner's popularity an issue in national congressional elections, they'll be asking independents to exchange his leadership for....Nancy Pelosi. In a contest between two incredibly unpopular choices, the incumbent has an edge, and the best marketing has an edge. The GOP has both.
C. Republicans and Independents-who-vote-Republican will not express their displeasure with incumbent Republicans by voting for Democrats.
D. District boundaries matter. Both in how many likely voters live in each district (favors the GOP) and how much campaign infrastructure can be organized for Congressional elections (favors the GOP).
E. You have to have electable Democratic candidates to run in those Republican-leaning districts, and they have to be able to organize campaign infrastructure.
The Republicans will maintain control of the US House of Representatives. They will actually increase their majority. The Senate will be under the control of the GOP, possibly to a supermajority. The President can get reelected, but only if Democrats nationwide start realizing how many disadvantages they are actually facing in 2012.