If the House can outsource impeachment, the deepest concerns of the Framers will become reality. Impeachment would have few limits and no political accountability. As a federal prosecutor, Mr. Mueller legitimately obtained information from a grand jury, wiretaps and other forms of surveillance unavailable to Congress. If Congress can secure these materials by simply commanding the executive branch to turn them over, it would tremendously augment its power.

Turnover of prosecutorial materials would allow Congress to hide behind the fact-finding and legal determinations of the other branches, thereby diminishing its own political accountability. Because the nation’s law-enforcement officials have concluded Mr. Trump has not committed any crimes, Democratic representatives cannot legitimately draft articles of impeachment accusing him of criminal conduct involving the same offenses of which he was cleared by the Mueller investigation. The House could impeach him for misconduct that doesn’t violate criminal statutes—say, abuse of power or inappropriate behavior. But lawmakers must be candid about what exactly the charge is.

Proceeding in such a fashion—not hiding behind criminal accusations that prosecutors have rejected—would require House Democrats to assume the full political risk for their impeachment efforts. Instead, they are pressing Mr. Mueller to testify, hoping he will say something beyond what is contained in his report, and to obtain his investigatory materials. By second-guessing the prosecutors and recasting Mr. Trump’s conduct as criminal-law violations, Democrats seek cover for their raw political push to unseat a president.