Several times Republicans have turned what would be slam dunk Senate races for a stand-issue Republican into tight or even uphill contests by nominating right-wing whackos. The New York Times reports that they've done the same thing in the House:

Now there are 40 Democratic incumbents remaining in districts won by Mr. McCain...
For a variety of factors, including fund-raising strength and the quality and ideological positioning of the Republican candidates, only 15 of the 40 districts are considered top targets by the National Republican Congressional Committee. Several others are rated competitive by nonpartisan analysts.
At least a handful of Democrats in the 40 districts are no longer considered to be as vulnerable as Republicans had hoped, largely because their preferred candidates were defeated by more conservative candidates in primaries.

I still think Republicans have a 50-50 chance or better to win the House. But they've made it harder on themselves because the party leadership can't control the base's tactical radicalism.

Of course, the flip side of this strategy is that, by nominating extremely conservative candidates, Republicans maintain very tight party discipline among the members they do elect. Democrats are much more prone to nominate moderates in swing districts or states. This means Democrats have more seats than they "should" have, but they also have a more fractious caucus.