The Upshot calculates the Democrats’ current chances of holding the Senate:
Every day, our computer churns through the latest polls and reams of historical data to calculate both parties’ chances of winning control of the Senate. Although the Democrats currently have a 51 percent chance, that doesn’t mean we’re predicting the Democrats to win the Senate — the probability is essentially the same as a coin flip.
Nate Cohn looks at the role incumbency plays:
Democrats’ hopes of keeping their Senate majority this November may well hinge on the ability of three of their incumbents to hold onto their seats deep in enemy territory: the South. To take the Senate and consolidate their control of Congress, Republicans need only extend their stranglehold on Dixie to Arkansas, Louisiana and North Carolina, all of which voted for Mitt Romney in the 2012 presidential election.
Yet the hopes of these three states’ incumbent Democratic senators — Mark Pryor, Mary Landrieu and Kay Hagan — are still alive. That may be surprising in light of the region’s lurch to the right, but it shouldn’t be: Incumbency is powerful. In the South, Democratic incumbents have won 85 percent of the time since 1990, and 77 percent since 2000.
The big Republican gains in the South have come mainly in open contests without an incumbent, often after a longtime Southern Democrat retires. Republicans have won 84 percent of open races in the South since 2000 — and three of their four losses came in Virginia and Florida, states that are different from the rest of the region.