Nate Silver has some suggestive evidence that the "generic ballot" polling question -- asking people whether they plan to vote for the Democratic or the Republican candidate for the House of Representatives -- may overstate the Republican vote. You should read his whole post, but there's a lot of reason to think some voters have some general desire to vote Republican but plan to vote for the specific Democrat in their district.
This makes a lot of sense given the differing nominating strategies the two parties have pursued. Democrats are obsessed with finding candidates who fit their districts, often recruiting anti-abortion, pro-gun candidates for rural districts. Republicans are increasingly nominating extremely conservative candidates everywhere, following the rising tide of tactical radicalism in the GOP.
It's quite natural, then, that there would be a lot of voters who like Republicans but find their Democrat candidate acceptable, or their Republican candidate unacceptable. In general, the importance of having a candidate who ideologically matches the views of the median voter is wildly overrated. But it's not worth nothing, and this could be a case where the Democrats' fealty to the old belief in representing the center pays dividends around the margin.