Bottom line: while we shouldn't expect any MSM pundit discussions about how Griffith's departure is a sign of the "narrowing" of the Democratic Party, this is still a noteworthy development. Just as the Republican Party's rightward and Southern shift has placed a burden on moderate Northeastern Republicans, so the Democratic Party's leftward and Northern shift has put pressure on moderate Southern Democrats. Now that the liberal Democrats are in charge - pushing their agenda and taking responsibility for the state of the Union - this pressure has become more salient. Griffith may or may not be the only Democrat to make an actual jump to the GOP, but his departure from the Democratic Party underscores the tension between the liberal leadership and many Southern moderates as the House prepares for a big health care vote.
As Cost writes, "The decades-long geographical and ideological sorting of both parties is ongoing." And as the northeast becomes more liberal (and hence Democratic) and the south becomes more conservative (and hence Republican), the real battles will be in purple places like the Mountain West.