In the last few days, conservatives have become agitated about Mitt Romney’s drop-off in the polls. So did they think the stumble was because of the ill-fated “47 percent” slip of the lip, or the hasty effort to gain a political edge after the death of an American ambassador in Libya, or more problematically, a campaign that can’t seem to stop pratfalling no matter what the news?

No, in their view, the mysterious drop can only be explained by the fact that the mainstream media have their collective liberal thumb on the scale, in terms of coverage and, more oddly, polling.

On Sunday, Mr. Romney’s running mate, Representative Paul D. Ryan, got right to the point.

“It goes without saying that there is definitely media bias,” Mr. Ryan told “Fox News Sunday.” “I think most people in the mainstream media are left of center and, therefore, they want a very left-of-center president versus a conservative president like Mitt Romney.”

And ostensibly tendentious coverage was cited last Wednesday in a letter addressed to the “Biased News Media” and sponsored by the Media Research Center, which defines its mission as “holding the liberal media accountable for shamelessly advancing a left-wing agenda.” The letter said in part: “This election year, so much of the broadcast networks, their cable counterparts and the major establishment print media are out of control with a deliberate and unmistakable leftist agenda.”