Loss of a seat indicates that the Senator was on the wrong side of the issues to many times.
In 2014; it general it would be the backing of Obama and the ACA.
There will need to be an issue in 2016 that will filet the Republicans in general.
At this point; I have not seen one appear.
This is pretty pointless. Not only is it foolish to predict an election almost a year out, but it's exceptionally foolish to predict one three years out.
Democrats hold a number of seats in red states that are up for election in 2014. Additionally, it's an off year election which tends to help Republicans. This means they are likely to lose some of them. As of last time I checked things were about even odds for the Republicans taking the Senate in 2014.
In 2016 Republicans hold a large number of seats in blue states and it's a presidential election year which tends to help Democrats. This means they are likely to lose some of them. The number of vulnerable Republican seats in 2016 is larger than the number of vulnerable Democratic seats in 2014, so that makes things worse.
The idea that we would have pinpointed an issue that will be salient in elections three years from now is like saying that in 2005 we knew what the 2008 election would swing on.
