From Political Wire:
Democrats need to pick up a net of four seats and the White House to control the Senate next year or a net of five seats. The current Senate class is the class of 2010 which, as we all remember, was a massive wipe-out year for Democrats as the Democratic holds in vaguely contested seats were enabled by allegations of witchcraft, outright craziness, and Teabagging extremism. Otherwise, the Republican wave swept up seats in states that typically swing about five points more Democratic than the nation as a whole.
There are two seats that in my mind lean heabily Democratic take-over (Wisconsin and Illinois), and then a bevy of seats that, in a neutral political environment are winnable by good campaigns (Ohio, Florida, New Hampshire, Pennsylvania) and another couple of seats that need a little bit of luck or a favorable national environment (North Carolina, Arizona if McCain retires, Indiana if the candidate is a teabagger). Two Democratic seats are at risk (Nevada and Colorado), I would put Nevada at more risk than Colorado. So best case Democratic scenario is a net plus seven or eight, decent case is net plus 6, and an oh-shitter is net even to net plus two as the 2018 map is ugly for Democrats as it is a 2012 class in an off-year electorate.
Getting Hassan to run increases the probability of a net pick-up and building a cushion for 2018.