Jonathan Chait: "The legislative dynamics in Washington are very simple. Gridlock exists because Obama and House Republicans cannot agree on legislation. If Obama and the House could agree on legislation, their deal would be approved by a Democratic-controlled Senate or by a Republican-controlled Senate. There are no plausible circumstances in which the Senate would block a deal struck between the House and Obama, because, whichever party controls the Senate, its ideological center will sit comfortably inside in the enormous space between Obama and the House Republicans.