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Think of the Congressional seats held by the president's party as an island, and the president's approval rating as the water that surrounds that island. As his approval rating declines -- as the water recedes -- more of his party's seats are exposed.

In 2010, the Democratic House losses were overwhelmingly concentrated in seats where President Obama won less than 55 percent of the vote in 2008 -- the places where his support was most marginal to begin with. As Obama's approval eroded during his first two years, House Democrats who had succeeded in 2008 in marginal or Republican-leaning districts could not survive the receding tide. Republicans in 2010 captured 22 of the 35 Democratic-held House seats that Obama won with less than 55 percent of the vote in 2008 and fully 36 of the 48 seats House Democrats had won in districts that voted for Republican nominee John McCain.

Obama's approval rating is even lower today than it was at the time of the 2010 election. That suggests that if Obama's support remains deeply depressed, the "water line" of Democratic House seats in danger in 2012 would extend more deeply into Democratic territory and include more seats that provided Obama a margin even greater than 55 percent last time. The New York-9 district, which Obama carried with 55.3 percent of the vote last time, may be a leading indicator of that risk. In 2010, Republicans captured only eight of 173 Democratic-held seats in which Obama's 2008 margin of victory exceeded his showing in the New York district. The magnitude of the GOP victory there on Tuesday suggests that many more of those seats could be at risk next year -- unless Obama finds a way to revive his fortunes.

Tuesday's results aren't prophecy: they don't guarantee Democrats will lose seats like that next year, or that Obama can't recover some of the support he has lost since 2008. But the New York results do foreshadow the risk that even House Democrats who once thought themselves safely on dry land could face if he does not.