Last week, Tom Steyer, the billionaire progressive donor, announced a $10 million campaign calling for President Trump’s impeachment, beginning with a television commercial running in all 50 states. Trump, the spot says, has “brought us to the brink of nuclear war, obstructed justice at the F.B.I., and in direct violation of the Constitution, he’s taken money from foreign governments and threatened to shut down news organizations that report the truth.” Appearing on screen, Steyer asks, “If that isn’t a case for impeaching and removing a dangerous president, then what has our government become?”

It’s a good question. Yet while most elected Democrats probably agree that Trump’s presidency is a nightmare, they’ve been largely reluctant to use the “I” word. The base wants impeachment — according to an August survey from the Public Religion Research Institute, 72 percent of Democrats support efforts to remove Trump from office. But inside the Beltway, calling for impeachment remains strangely taboo.

Some members of Congress are awaiting the results of the investigation being conducted by Robert Mueller, the special counsel, and the case for impeachment may become stronger when his inquiry is complete. Yet whatever Mueller discovers, we have credible reasons for impeachment right now. The Constitution dictates that presidents be impeached for “treason, bribery, or other high crimes and misdemeanors.”

But as the Harvard Law scholar Cass Sunstein, author of the recent book “Impeachment: A Citizen’s Guide,” told me, that doesn’t mean Congress can impeach only a president who is caught breaking the law. “Crime is neither necessary nor sufficient,” said Sunstein, who emphasizes that his book is not about Trump. “If the president went on vacation in Madagascar for six months, that’s not a crime, but that’s impeachable.”