Foreign countries are always trying to steal U.S. secrets, and they sometimes succeed. In this case Mrs. Feinstein tweeted over the weekend that the FBI approached her five years ago with concerns about an “administrative” staffer in her San Francisco office with “no access to sensitive information.” She said she “learned the facts and made sure the employee left my office immediately.”

This is what the FBI should do, and the question Mr. Trump should ask is why the bureau didn’t treat him as a potential President with the same customary courtesy. The FBI claims it had concerns beginning in spring 2016 that low-level Trump campaign staffers Carter Page and George Papadopoulos were colluding with Russians. Yet rather than give the Trump campaign the usual defensive briefing, the FBI launched an unprecedented counterintelligence investigation into a presidential campaign, running informants against it and obtaining surveillance warrants. The country is still enduring the polarizing fallout from that decision through special counsel Robert Mueller’s probe.

This disparate treatment is evidence that the FBI abused its authority in 2016, whether or not it acted with political bias. The bureau routinely warns politicians, campaigns and others about espionage threats. In Mrs. Feinstein’s case, the bureau had located an actual spy—and then went directly and discreetly to the Senator.

In Mr. Trump’s case, the FBI by its own admission was operating on nothing more than suspicions (many from the Clinton campaign-financed Steele dossier), and to this day the bureau has never presented definitive evidence of the campaign’s collusion with Russia. Yet it launched a full investigation that it didn’t disclose to Congress.

Mrs. Feinstein is also doing nobody a favor by downplaying this breach. She claims the driver never had access to “sensitive” information, but the infiltration of the staff of a Senator who serves on the Senate Intelligence Committee is no small matter. Who knows what the spying staffer was able to hear and report to China over the years?

The Russia probe has become such a partisan Beltway fixation that it obscures larger issues of governance that will outlast Donald Trump and Dianne Feinstein.