House Intelligence Committee Chairman Devin Nunes (R-Calif.) at the Capitol in Washington. (J. Scott Applewhite/AP)

IF REPUBLICANS are trying to uncover serious FBI misconduct, they aren’t acting like it. Instead, every move President Trump and his lackeys in Congress have made lately suggests they are constructing, at the expense of the reputation and effectiveness of federal law enforcement, a slanted, a-factual narrative to discredit any negative stories that might emerge from the investigation of special counsel Robert S. Mueller III.

The latest is a Thursday meeting Mr. Trump arranged between top Justice Department officials and two Republican members of Congress to review highly sensitive department materials relating to the beginnings of the Russia investigation, materials that the department has strongly resisted sharing with lawmakers. One of those members, House Intelligence Committee Chairman Devin Nunes (Calif.), has already proved himself willing to abuse his office to help the president. Democrats have been excluded from the meeting, even though they want a seat at the table.

The Justice Department typically offers identical but separate briefings to House Democrats, which should happen in this case. Alternatively, Justice could meet with the full “Gang of Eight,” a bipartisan group that includes House and Senate leaders and receives intelligence briefings. Neither option seems to have occurred to the White House. Trump press secretary Sarah Huckabee Sanders said Tuesday that Democrats were excluded because they had not asked previously to review the specific documents that Republicans had.

Passing secret information to pro-Trump spinners in a session closed to anyone unsympathetic to the president is not legitimate congressional oversight. There is no reason to exclude Democrats if Republicans intend to be honest.

Take Mr. Trump’s wild conspiracy theorizing seriously for a moment. Imagine that the Obama administration in 2016 used the FBI to infiltrate a GOP presidential campaign with the intent to undermine it. If secret Justice Department documents actually suggested such a plot, why would Mr. Trump want to prevent Democrats from seeing them, forcing them to admit that they have been wrong to question his claims? As it stands, Democrats can fairly point to the closed meeting itself as evidence that congressional investigative powers are being misused to build a self-serving GOP tale about FBI perfidy.

It should go without saying — but apparently does not — that the conspiracy theory is absurd. If the FBI’s goal had been to undermine Mr. Trump’s campaign, why would the Justice Department have sat on the information it had about that campaign’s ties to Russia, even as then-FBI Director James B. Comey publicly lambasted Hillary Clinton for poor judgment on her private email server? Presumably the plotters would have wanted to prevent Mr. Trump’s election rather than merely undermine him after he was elevated to the Oval Office.

The president’s assertions seem to get more baldly implausible by the day. On Wednesday, Mr. Trump tweeted that the “Criminal Deep State” is “caught in a major SPY scandal the likes of which this country may never have seen before!”

The most natural inference from such rhetoric is that it is intended to hide guilt on the part of the president or his senior staff. But some combination of incompetence and malevolence is also a possible explanation. The only way to know is to allow Mr. Mueller to complete his investigation.