The simplest explanation for Barack Obama’s problems with his base is the Senate. While many on the left are unhappy with the President’s foreign policy and anti-terrorism policies, the vast majority of liberal discontent has resulted from domestic policy compromises necessitated by Senate rules — in particular, the size of the first stimulus, the death of the public option, and the tax cut deal Obama just reached with Republicans. In each case, Obama’s critics have invoked Green Lantern-style arguments that downplay or ignore the need to get enough votes in the Senate. So why isn’t Obama framing his compromises as necessary under current Senate rules and reiterating his call for reform of the filibuster? (See Jamelle Bouie’s tweet yesterday for a suggestion to liberals along these lines.) Isn’t that the kind of coalition maintenance strategy that Brad DeLong wants?