He is citing redistricting as the reason for this:

US Representative Barney Frank, the state’s highest-profile congressman and one of the nation’s leading liberal voices after being among its first openly gay elected officials, announced today that he will not seek reelection next year.

The Newton Democrat faced the prospect of a bruising reelection campaign next year after surviving a brutal battle in 2010. He also would have run in an altered district that retained his Newton stronghold but encompassed more conservative towns like Walpole.

The PV on the new district was about +10 PV for the Dems, as opposed to about +18 PV in the old one, so the idea of a “bruising” reelection campaign is a joke.

I will take him at his word, that redistricting was the straw that broke the camel’s back, but I also think that it is also an indication of the fact that he does not believe that Democrats will take back the house in 2012, and I also get the sense that he is sick and tired of fighting the Obama administration over financial regulation, where his own admittedly weak tea is watered down.

I’ll miss him.