Just a note on how large the playing field is for Republicans right now: Last week, the top-notch Cook Political Report rated David Obey's seat as a "likely" Democratic hold. Cook lists 35 "likely" Democratic seats, and 62 seats which are more likely to flip Republican.
Did Obey see polling that showed his position more dire? Perhaps. But maybe a bigger factor is that he thinks the Democrats will lose the House and he doesn't want to go back into the minority. I wonder how many of his colleagues are having similar thoughts right now.