One of the favorite themes of mainstream media, the Obama campaign (when not accusing Mitt Romney of being a flip-flopper) and liberal pundits is that the Republican Party is in the grips of the right wing. But the irony is rich here: While the GOP is nominating the least right-wing of the presidential candidates, the Democratic faithful are purging moderates from their party.

If you don’t believe me, go read the New York Times (just this once):

The defeat of two conservative House Democrats by more liberal opponents in Tuesday’s Pennsylvania primary illustrates the strong hold the new health care law still has over committed Democratic voters and foreshadows an even more polarized Congress next year in the aftermath of the latest round of redistricting . . . .

Representatives Jason Altmire and Tim Holden both lost in primaries to opponents who joined together with activist groups to pummel the veteran lawmakers over the opposition to the new health care law and climate change legislation — positions they had used to their advantage in the past to show their independence from President Obama and the Democratic Party.

“A lot of us thought of his record as his strength,” said Hugh M. Reiley, the chairman of the Schuylkill County Democratic Party, referring to Mr. Holden. “He was not falling prey to all that party bickering. He was able to reach across the aisle.”

“Last night, the Democratic Party became more liberal,” he added.

Indeed this process has been going on for some time. With the exception of Sen. Joe Lieberman (I-Conn.), who began as a Democrat but is now an independent, there is no hawkish senator in the mold of Sen. Sam Nunn (D-Ga.). The moderate senator with the highest profile in recent years, Sen. Evan Bayh (D-Ind.), left the Senate in 2010 complaining about polarization and gridlock.

In the House, the Times observes: “The demise of the conservative ‘Dixiecrats’ in the 1960s and ’70s made the Democratic Party more liberal, and Republicans have moved even further to the right than Democrats have moved to the left, the studies show. Elections like Tuesday’s suggest Democrats may be taking the Republicans’ cue, driven by the same activist forces that pushed them rightward.”

Recall the famous Blue Dog Democrats who were fiscally responsible and socially conservative? They’ve left or been defeated. (“With the defeat of Mr. Altmire and Mr. Holden, a Blue Dog coalition of conservative Democrats that peaked in 2010 at 54 dipped prospectively to 23.”)

And of course the president himself, a far cry for the “Third Way” of Bill Clinton, has pushed his party leftward and is running a presidential race based on pumping up his base (e.g., women, especially) with naked appeals to gender, class and ethnicity. The same pundits who sing the praises of “maverick” (read: unconservative) Republicans and bemoan the rightward shift in the GOP also cheer the demise of Obamacare opponents. Aren’t they worried about polarization on the Democratic side? Apparently not.

There is little doubt that given his druthers (“more flexibility’) in the second term, Obama would pull the country further leftward. But the same is true of the Democratic Party that he leads. Where is there a place for the next Ben Nelson or Blanche Lincoln (Democrats capable of winning in red states)? And more important, are truly moderate voters — the sort worried about the debt, wary of tax hikes and uncomfortable with sacrificing energy development at the altar of environmental extremism — really welcomed in the Democratic Party? (In other words the sort of Democrats bamboozled in 2008 into thinking Obama was a middle-of-the-roader.) It sure doesn’t seem like it.