There are a whole host of reasons why the Democrats might lose this seat: an unappealing candidate, how few things Obama has actually delivered, the cost of the health-care bill, the fact Massachusetts, basically, already has universal health care. But this along with the Democrats losing in governors' races in Virginia and New Jersey shows that it is just an awful time to be an incumbent. The electorate is, understandably, both nervous and impatient.
But if the Democrats drop below sixty votes in the Senate, the number needed to break a filibuster, they are going to find it very hard to pass their health-care bill without some procedural chicanery. This means that the project Obama chose to spend vast amounts of political capital on this year could come to nothing.
PS As Alex cautions, the result isn’t actually in yet.