The thing is, we know that many last-minute undecided voters ultimately decide not to vote.  But it doesn’t follow that these are apathetic voters. In fact, you can explain much of the above if it is the case that the voters who stayed home in 2012 were adults who fit the profile of likely voters, disapproved of the job that the president did, and ultimately decided that they didn’t care for the Republicans either and opted to stay home. 

In this sense, I think the large number of undecided voters — who almost certainly disapprove of the president by large margins — are a potential red flag for Republicans. At this point, what more can Republicans do to convince them to make up their minds? Mark Warner has been stuck in the high 40s/low 50s for several months now. In theory, Ed Gillespie should be making a race of it by now.  Yet he remains mired in the high 30s (although he has closed the gap somewhat).  There seems to be a substantial chunk of the Minnesota electorate that isn’t prepared to commit fully to Al Franken, yet isn’t excited about Mike McFadden.

If these voters ultimately opt disproportionately to stay home, it would transform an electorate where the president has a 42 percent job approval into one where he has a 46 percent job approval.  This probably wouldn’t be enough to save the Senate: Democrats who trailed would still lose, albeit by small margins.  But it would probably cap Republican gains in the House, and would probably transform an opportunity for a huge GOP night in the Senate into a modest wave of six or seven seats.