As I’ve been telling people all week here in Pittsburgh, there’s ample reason for Democrats to be worried — perhaps deeply so — about 2010. Without major intervening events like 9/11, the party that wins the White House almost always loses seats at the midterm elections — since World War II, an average of 17 seats in the House after the White House changes parties. Democrats have substantially more seats to defend than Republicans, particularly in the House. They appear to face a significant enthusiasm gap after having dominated virtually all close elections in 2006 and 2008. And the economy and health care are contingencies that could work either way, but which probably present more downside risk to Democrats than upside over the next 12-18 months, particularly if some version of health care reform fails to pass. While the Democrats are not extraordinary likely to lose the House, such an outcome is certainly well within the realm of possibility (I’d put the chance at somewhere between 1-in-4 and 1-in-3). The Senate picture is a bit brighter for them, but they are probably more likely now to lose seats in the chamber than to add to their majority, in spite of the spate of Republican retirements in Ohio, Missouri and other states. In a wave-type election, a net loss of as many as 4-6 seats is conceivable.