For the political cognoscenti, the dominant story line in July has been the Barack Obama campaign’s mastery of the subterranean arts of opposition research to mar Mitt Romney’s Bain chance.

As Time’s Mark Halperin, the co-author of Game Change, put it this week, “They have parceled out their opposition research in a manner both strategic and tactical, selecting specific news organizations at times of their choosing to maximize the drip-drip-drip.” Former Al Gore campaign adviser Carter Eskew mused in an online column for The Washington Post, “Romney’s dark period isn’t over…Makes me wonder if more opposition research is on its way.” Even Romney himself, as he concocted ever more creative justifications for not releasing his tax returns prior to 2010, complained, “You can never satisfy the opposition research team of the Obama organization.”

Despite all the attention lavished on the supply side of opposition research (those crafty Obama operatives), there has been scant discussion of the demand side (the media outlets that publish oppo). It strains credulity that the firestorm of recent anti-Bain articles was entirely sparked by the journalistic equivalent of spontaneous combustion.

Of course, we don’t know for sure, since no reporter is gleefully ballyhooing his or her role in serving as a transmission belt for the oppo research swag provided by the Obama campaign—and also because the standard journalistic formulations, which hide sourcing in the passive voice, leave open the possibility that the reporter benefited from an oppo tip. Rummaging through the pile of anti-Bain stories, it is easy to come up with many examples of who-knows-where-it-came-from sourcing. For example, an exposé last week about contradictions dating back to Romney’s 2002 efforts to prove that he was a Massachusetts resident so that he could run for governor included the phrase, “the testimony, which was provided to The Huffington Post.” Another mid-July story about Romney’s SEC documents contains the cryptic, though not uncommon, sourcing explanation, “obtained by Mother Jones.”

(Every time I mention The Huffington Post, I feel obligated to point out that as part of her 2011 alliance with AOL, Arianna Huffington shut down Politics Daily, where I was a senior correspondent).

Obviously, there is no way for readers to know precisely who provided the contradictory testimony to The Huffington Post or exactly how Mother Jones obtained the politically damaging SEC documents. For all I know, those publications devoted weeks of investigation to these stories. (I don’t believe in grilling reporters about their sources, so I haven’t asked them.) And to be fair, there is equal mystery about the provenance of pro-Romney stories. A Fortune article defending Romney’s account of his tenure at Bain contains the opaque look-what-we-found explanation, “according to confidential firm documents obtained by Fortune.”

Romney, though, is the target of most opposition research for the simple reason that it is invariably the challenger who is ill-defined in the minds of the voters. For all the efforts by the right-wing press to once again dredge up Jeremiah Wright and Bill Ayers, anti-Obama voters do not need bygone political history to convince themselves to oppose the president’s reelection. In fact, I cannot recall a single example of an incumbent president losing votes because of anything he did prior to entering the White House.

All this brings us to a journalistic dilemma that needs to discussed in the midst of this already bitter president race in, which the neutrality of the press corps will be repeatedly questioned: What are the proper rules for handling opposition research? Is it enough, as many reporters and editors believe, merely to independently verify the accuracy of the over-the-transom material? Is there a meaningful difference between documents that are directly provided by campaign operatives and those that are sanitized by being passed through other means? And, finally, should self-serving political leaks be handled any differently than, say, explosive information provided by a disgruntled bureaucrat at the Commerce Department?

Walter Shapiro just chronicled his ninth presidential campaign. He writes the “Character Sketch” political column for Yahoo News. Follow him on Twitter @WalterShapiroPD.