Snowe, the Senate's most liberal Republican, found herself in a familiar spot Thursday as the only member of her party to join with Democrats on a politically freighted vote. This time, it was a vote to affirm an Obama administration directive requiring employers to provide contraception coverage to their workers regardless of religious or ethical concerns.

The vote, originally demanded by Republicans in a political battle that Democrats came to embrace, provided ample fodder for political ads but had nothing to do with an underlying highway bill. That measure continues to twist in the wind despite widespread support, trapped in a divisive, polarized Senate that rarely seems to legislate and often seems incapable of tackling politically challenging problems.

So Snowe, 65, is leaving at the end of the year, voicing frustration that the Senate is simply too polarized and that she doesn't know whether she could be "productive" in a fourth Senate term.