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 expos√© 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?

After three decades covering presidential politics, I am surprised to find myself on the prissy, schoolmasterish side of the ledger on all these questions. I believe readers deserve to know when purported journalistic exclusives have their roots in partisan oppo research. It strikes me as legalistic hair-splitting to make a distinction between being handed files by a political source and being instructed to inspect all documents in drawer C-1163-B at the SEC. And, yes, I do think there is a difference between a political campaign orchestrating a front-page investigation that morphs into an attack ad and a whistleblower alerting the press over wasteful contracting by the Commerce Department.

It all comes down to the bonds of trust between reporters and readers. If a publication is knowingly being used for partisan gain, it should disclose the contours of the transaction. Please understand: I am not a Pollyanna demanding that you have to identify a specific deputy press secretary or insist that every conversation with a campaign official must be on the record. It is enough to say in print something like, “Documents cited in this article were provided by Democratic sources with a partisan motivation to embarrass Romney.” Or, perhaps, “Initial guidance for this article and specific instructions on how to obtain documents were provided by Republican sources whose goal is to defeat the president in November.”

Part of the rationale for transparency is journalistic self-preservation. These days, virtually every investigative story about a candidate’s background is cynically assumed to be based on planted evidence. As oppo research comes out of the shadows (Romney’s campaign manager Matt Rhoades was a dirt-digger during George W. Bush’s 2004 reelection race), it won’t be long before campaigns start publicly bragging about story placement. In fact, it already happened in the 2010 Connecticut Senate race. GOP contender Linda McMahon claimed credit for a New York Times story documenting how Democratic rival Richard Blumenthal inaccurately portrayed himself as a Vietnam veteran. In that case, Bill Keller, then the executive editor of the Times, said, “The idea that the story originated or was sold to us by the McMahon campaign is just plain false.” But if readers never know when oppo research is the source of a story, they will always suspect that it is.

The standard argument against full disclosure of opposition research is that campaigns will not make the information available to the media unless they can do it without leaving fingerprints. Really? Do you think 21st-century campaign operatives would be so reticent about tarring their opponent that they would prefer that the information remain secret rather than be identified as the source? In truth, campaigns are using the credibility of the news media to put an imprimatur of objectivity on opposition research that would come out in any case.

In an Internet era—for those who haven’t noticed—political organizations are quite capable of self-publishing. Last week, a pro-Obama super PAC, American Bridge 21st Century, launched a website called VeepMistakes.com, which contains 1,300 pages of oppo research on three leading contenders to be Romney’s running mate (Rob Portman, Tim Pawlenty, and Marco Rubio). Of course, these items are partisan spin: (Page 345) “Pawlenty Invented His Reputation For Being Tough On Illegal Immigration.” But everybody reading the American Bridge dossiers—including reporters profiling Romney’s eventual VP—understands the political pedigree of the material.

News organizations—even in an era of parched resources—should go all out in probing Romney’s business history, especially since his career in public office is limited to just four years. (Odd how Romney has spent more of his life running for president than being governor). Obviously, Obama’s record as president is fair game as well. Ideally, the news media should do its own investigative work rather than turning to political gumshoes for assistance.

But if the Obama or Romney campaigns do get involved (and these days their political research budgets likely dwarf those of the networks and major newspapers), acknowledge the assistance in print. Truth-in-packaging is such a bedrock journalistic principle. And maybe honesty by the media (and, yes, a bit of self-flagellation) is needed to explain to the voters exactly how opposition research works in presidential politics.

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