Speaker Nancy Pelosi has directed committees investigating President Trump to “proceed under that umbrella of impeachment inquiry,” but the House has never authorized such an inquiry. Democrats have been seeking to impeach Mr. Trump since the party took control of the House, though it isn’t clear for what offense. Lawmakers and commentators have suggested various possibilities, but none amount to an impeachable offense. The effort is akin to a constitutionally proscribed bill of attainder—a legislative effort to punish a disfavored person. The Senate should treat it accordingly.

The impeachment power is quasi-judicial and differs fundamentally from Congress’s legislative authority. The Constitution assigns “the sole power of impeachment” to the House—the full chamber, which acts by majority vote, not by a press conference called by the Speaker. Once the House begins an impeachment inquiry, it may refer the matter to a committee to gather evidence with the aid of subpoenas. Such a process ensures the House’s political accountability, which is the key check on the use of impeachment power.

The House has followed this process every time it has tried to impeach a president. Andrew Johnson’s 1868 impeachment was predicated on formal House authorization, which passed 126-47. In 1974 the Judiciary Committee determined it needed authorization from the full House to begin an inquiry into Richard Nixon’s impeachment, which came by a 410-4 vote. The House followed the same procedure with Bill Clinton in 1998, approving a resolution 258-176, after receiving independent counsel Kenneth Starr’s report.

Mrs. Pelosi discarded this process in favor of a Trump-specific procedure without precedent in Anglo-American law. Rep. Adam Schiff’s Intelligence Committee and several other panels are questioning witnesses in secret. Mr. Schiff has defended this process by likening it to a grand jury considering whether to hand up an indictment. But while grand-jury secrecy is mandatory, House Democrats are selectively leaking information to the media, and House Republicans, who are part of the jury, are being denied subpoena authority and full access to transcripts of testimony and even impeachment-related committee documents. No grand jury has a second class of jurors excluded from full participation.

Unlike other impeachable officials, such as federal judges and executive-branch officers, the president and vice president are elected by, and accountable to, the people. The executive is also a coequal branch of government. Thus any attempt to remove the president by impeachment creates unique risks to democracy not present in any other impeachment context. Adhering to constitutional text, tradition and basic procedural guarantees of fairness is critical. These processes are indispensable bulwarks against abuse of the impeachment power, designed to preserve the separation of powers by preventing Congress from improperly removing an elected president.