Sean Hannity of Fox News Channel appears at National Harbor in Maryland on March 4, 2016. (Carolyn Kaster/Associated Press)

Why is Sean Hannity pushing the discredited conspiracy theory that a slain staff member of the Democratic National Committee was tied to the leak of emails from the DNC in 2016? Well, he told the world himself on Twitter on Sunday.

If the conspiracy theories around Seth Rich are true — that is, that he had some hand in the leak — that means that the hack of the DNC wasn’t a function of the Russians, as the government has determined. And that, trickling outward in Hannity’s argument, means that there’s no need to investigate how Russia meddled in the election — and that means that the shadow hanging over President Trump must dissipate.

The theories Hannity presents aren’t true, as our David Weigel made clear  Saturday. The case that Hannity has sloppily prosecuted over the past week is full of holes obvious to an impartial observer, but Hannity is not now and never has been an impartial observer. That’s not new.

What’s new here is that Hannity is making his strategy so explicit in that tweet. “If Seth was wiki source, no Trump/Russia collusion,” he writes. If X then Trump is innocent — therefore, no matter what, X, even if it means accusing a slain man of something for which there’s no evidence and even if it means tossing chum into another of the shark-filled cesspools of online conspiracy. Especially then, really.

Seth Rich came to D.C. to pursue a career in politics and last worked at the Democratic National Committee. Rich was shot dead on July 10, 2016. Here is what's known about the murder. (Claritza Jimenez/The Washington Post)

One of the remarkable side effects of universal access to information is how it has bolstered the human tendency to embrace information that reinforces our existing beliefs. Clearly the value in providing interconnected access outweighs the erosion of rational argument, but that erosion is substantial and disconcerting.

The Trump era has overlapped with the blossoming of a number of questionable rhetorical practices, not the least of which is the practice of responding to any critique of Trump with a tangentially similar critique of one of his political opponents, usually Barack Obama or Hillary Clinton. Someone, somewhere dubbed this “whattaboutism” — as in, “What about what Obama did?” CNN hired several of the world’s preeminent practitioners of whattaboutism in Jeffery Lord and Kayleigh McEnany, who will deflect nearly any negative news about Trump by pointing out some often-less-severe or unrelated offense by his predecessor.

Now here’s the thing. Often (although not always with Lord or McEnany) this is an innocent response, predicated on the assumption that Obama and Clinton were inherently less trustworthy than Trump. Normally, it’s then incumbent on outside interlocutors to clarify how and where what Trump did is different from what others did, but, again, the Internet always provides someone willing to blur that line. Sometimes that blurring is itself innocent in intent; often, it’s deliberately misleading.

Hannity’s valiant WikiLeaks efforts demonstrate a correlated facet of whattaboutism: Any one small question invalidates the overall claim.

We’ve seen Trump do this, too, as when his claims about Obama wiretapping Trump Tower evolved into not being about explicit wiretapping as such and then into being about whether the government unmasked the identities of people who’d been in monitored conversations with foreign actors, an act that Trump called a “massive, massive story” — in fact, it was the “real story,” according to Trump, although experts strongly disagreed. But it didn’t matter: That was the detail on which the whole tricky Trump-did-nothing-wrong argument hinged, and it was apparently sturdy enough for anyone who wanted to climb aboard.

Notice that, if the question of wiretapping is a tree, this is a branch off a branch off a branch — far from the central issue. That’s where Hannity is with his ridiculous Seth Rich conspiracy theory. The Russia mess is a sprawling investigation that, The Washington Post reported Friday, is now targeting a senior official within the Trump administration. It extends well beyond the hacking of the DNC and into the hacking of Clinton campaign chairman John Podesta’s email and, more broadly, into any ways in which Russia tried to leverage its relationships to tilt the playing field toward Trump in 2016. Even if Hannity were right about the DNC angle — and, we can’t say this enough, there’s no indication that he is — it would do no more to unwind the broader investigation than finding that Al Capone paid taxes one year would render Eliot Ness’s efforts impotent.

It is absolutely critical that the media dig into stories and evaluate the claims that are made. If Hannity were doing that, all the more power to him. But that’s not what he’s doing. He’s amplifying a discredited voice solely for the purpose of making it heard and, in the tweet above, embracing uncritically anyone else who wants to bolster his point.

Why? Hannity’s point is to have that one thing, that one whattabout, that allows himself and those interested in joining him to preserve confidence in Trump.

The only thing is, he’s not supposed to tell people that’s what he’s doing.