KIMBERLEY STRASSEL: The Fusion Collusion: Democrats are trying to protect the firm’s secrets—so the GOP should keep digging.

Washington is obsessed with the word “collusion” but has little understanding of its true meaning. The confusion might explain why D.C. has missed the big story of collusion between Fusion GPS and the Democratic Party.

To read the headlines, a poor, beleaguered opposition-research firm was humiliated and constitutionally abused this week by partisan Republicans on the House Intelligence Committee. Fusion’s lawyers sent a 17-page letter to the committee’s chairman, Rep. Devin Nunes, accusing him of misdeeds, declaring his subpoenas invalid, and invoking a supposed First Amendment right to silence. Yet the firm’s founders, the story went, were hauled in nonetheless and forced to plead the Fifth. “No American should experience the indignity that occurred today,” Fusion’s lawyer, Joshua Levy, declared.

Fusion is known as a ruthless firm that excels in smear jobs, but few have noticed the operation it’s conducting against the lawmakers investigating it. The false accusations against Mr. Nunes—that he’s acting unethically and extralegally, that he’s sabotaging the Russia probe—are classic.

This is a firm that in 2012 was paid to dig through the divorce records of a Mitt Romney donor. It’s a firm that human-rights activist Thor Halvorssen testified was hired to spread malicious rumors about him. It’s a firm that financier Bill Browder testified worked to delegitimize his efforts to get justice for Sergei Magnitsky, a lawyer beaten to death in a Russian prison.

It’s the firm behind the infamous “dossier” accusing Donald Trump of not just unbecoming behavior but also colluding with Russia. Republicans are investigating whether the Fusion dossier was influenced by Russians, and whether American law enforcement relied on that disinformation for its own probe.

But Fusion’s secret weapon in its latest operation is the Democratic Party, whose most powerful members have made protecting Fusion’s secrets their highest priority.

The obvious inference is that Fusion’s secrets are their secrets, and they’re devastating.