I’m thinking about instituting a “Corzine Rule” for politics, which would go something like, “Any time a well-known incumbent is polling in the low 40s in head-to-head matchups, consistently, for a long stretch of time, a year before Election Day, in a race with no major third-party factor, that incumbent is toast.”

We saw this in New Jersey last year. And there are quite a few incumbents up for reelection in 2010 who are in similar situations: Ted Strickland in Ohio, Harry Reid in Nevada, Michael Bennet in Colorado. A few days ago, this list would have included Dodd, Dorgan, and Ritter, of course.

But the senator who’s looking really Corzine-y right about now? Arkansas’s Blanche Lincoln. Her percentage in the polls, head-to-head against state legislator Gilbert Baker, since September: 44, 39, 41, 41, 42, 39.

You could also make a case for Arlen Specter in Pennsylvania: 44, 42, 44, 40, 42, 42.

And while he doesn’t quite qualify as an incumbent, since he’s a House member who represents half the state, New Hampshire Senate candidate Paul Hodes is in similar circumstances.

Obviously, those embattled incumbents could be saved by some amazing twist of fate – their challenger could get caught in a massive scandal, or the previously unpopular politician could rush into a burning building and save puppies and orphans from certain doom or something. But that doesn’t happen that often . . .

UPDATE: A reader makes the case for Maryland governor Martin O’Malley, citing his not-so-hot approval rating, but I’d prefer to see how O’Malley looks in potential head-to-head matchups. Right now, it seems the hopes for a GOP win in Maryland hinge on a comeback bid from former governor Robert Ehrlich . . .