Bill Clinton was the first president to embrace the “permanent campaign,” meaning a White House always in full election mode. House Speaker Nancy Pelosi has now made her own dubious contribution to American history, by setting up the possibility of a Congress in permanent impeachment mode.

The mechanics of impeachment almost always promise a clash between a president of one party and a Congress dominated by another. Even so, by forgoing a full House vote to authorize an impeachment inquiry, Mrs. Pelosi has amped up the partisanship....