Think of the 2016 Trump-Russia narrative as two parallel strands—one politics, one law enforcement. The political side involves the actions of Fusion GPS, the Hillary Clinton campaign and Obama officials—all of whom were focused on destroying Donald Trump. The law-enforcement strand involves the FBI—and what methods and evidence it used in its Trump investigation. At some point these strands intersected—and one crucial question is how early that happened.

What may well have kicked off both, however, is a key if overlooked moment detailed in the House Intelligence Committee’s recent Russia report. In “late spring” of 2016, then-FBI Director James Comey briefed White House “National Security Council Principals” that the FBI had counterintelligence concerns about the Trump campaign. Carter Page was announced as a campaign adviser on March 21, and Paul Manafort joined the campaign March 29. The briefing likely referenced both men, since both had previously been on the radar of law enforcement. But here’s what matters: With this briefing, Mr. Comey officially notified senior political operators on Team Obama that the bureau had eyes on Donald Trump and Russia. Imagine what might be done in these partisan times with such explosive information.

And what do you know? Sometime in April, the law firm Perkins Coie (on behalf the Clinton campaign) hired Fusion GPS, and Fusion turned its attention to Trump-Russia connections. The job of any good swamp operator is to gin up a fatal October surprise for the opposition candidate. And what could be more devastating than to paint a picture of Trump-Russia collusion that would provoke a full-fledged FBI investigation?