I understand that Republicans would rather talk about a controversial newspaper ad than Iraq policy. I even understand why the right wants to demonize an effective and successful liberal activist organization.

But the problem in the wake of last week’s “Betray Us” ad from MoveOn.org is that the GOP just doesn’t know when to quit. They saw an opportunity, took it, and scored a few cheap political points. Instead of, ahem, moving on, conservatives keep going to the well, to the point that it’s now kind of embarrassing.

Last week, Rudy Giuliani suggested MoveOn shouldn’t have free-speech rights. John McCain suggested MoveOn be thrown out of the United States. This week, the Vice President is not only attacking the group; he’s also alleging some kind of conspiracy involving the New York Times. McCain has now incorporated an enlarged poster version of the “Betray Us” ad into his stump speech, and brings his new favorite prop everywhere he goes.

It’s reached the point that the ranking Republican on the House Oversight Committee wants to launch a congressional investigation into whether the NYT violated election law by selling the ad at a reduced rate. Seriously.

Rep. Tom Davis (R-Va.) asked committee Chairman Henry Waxman (D-Calif.) Tuesday to convene a hearing on the issue, saying that any rate change “for political advertising could constitute an unlawful campaign contribution.”

Now, in the broader context, Davis is running for the Senate, so he has to go out of his way to demonstrate his capacity for nuttiness to the GOP base, but this just isn’t healthy. Criticizing the ad is one thing; letting the ad drive the Republican Party into madness is something else.

In case there’ any lingering confusion here, the charges have no merit.