The big danger for President Trump in the second half of his term lurks in the Senate. That’s an irony, in that the upper chamber is where the Republicans just gained ground despite the blue wave that swept the GOP out of power in the House. The risk, though, is well marked in Saturday’s Times by the doyenne of liberal journalists, Elizabeth Drew.

Ms. Drew writes of impeachment’s “inevitability.” We wouldn’t gainsay it. We’ve long taken for granted that were the Democrats to accede to the leadership of the 116th House, impeachment would be their priority. The new House hasn’t even been sworn, and already the Democrats are hiring staff to investigate the president on every front.

In January 1974, as the House prepared to act on President Nixon, the Times reckoned it was “profoundly important” that the Judiciary Committee “approach this grave question in the proper spirit.” It would be but the second presidential impeachment, which the Times called “patently not a narrow partisan issue.”

Hmmmm. The first presidential impeachment pitted the Republicans and Lincoln loyalists against a Democrat who had served as vice president under Lincoln, a Republican. Johnson and Lincoln fetched up in the same party only for the 1864 election, when the War Democrats joined with the Republicans in the National Union Party.

Johnson was impeached for, in the main, firing Lincoln’s war secretary, Edwin Stanton, in violation of a law, the Tenure of Office Act, that would later be repealed and later still deemed by the Supreme Court to have been unconstitutional. There was no special prosecutor; the House voted its charges only three days after Stanton was cashiered. Not one Democrat voted to convict Johnson.

Nor, five generations later, did a single Democrat vote to convict President Clinton. Not for either obstruction or perjury. Johnson had been acquitted by one vote. Clinton was cleared roundly. So if the House acts against Mr. Trump in a “patently” non-partisan way, it would be the first such impeachment in history.

Which brings us to the Senate. Our guess is that a House bent on impeaching Mr. Trump won’t have a hard time finding an excuse. One of the charges it threw against Andrew Johnson was speaking ill of the House itself. On that precedent, the House could seek to oust Mr. Trump for his jibes on the Twitter.

The danger is a collapse of the Senate. That is precisely what happened to Nixon — and when the House was merely considering impeachment. It happened with astonishing swiftness after the Supreme Court on July 24, 1974, ordered Nixon to produce tape recordings made in his office.

On July 27, the Judiciary Committee voted out three articles against the president — abuse of power, contempt, and obstruction. It cleared Nixon for bombing Cambodia and failing to pay taxes. On August 7, three Republicans — Senators Barry Goldwater and Hugh Scott and Congressman John J. Rhodes Jr. — slithered into the Oval Office.

The trio told Nixon his support in the Senate had collapsed, even among the GOP. The next day, Nixon announced he would resign on the morrow. Without Nixon even getting impeached and the Senate hearing a single witness, the solons overturned an election in which Nixon had won 49 states.

We don’t mind saying that we hope this doesn’t happen to President Trump. America hasn’t yet seen in his case anything close to the particulars adduced against Nixon. Nor has anything like a due impeachment process yet been begun in the House. The moral of the story for Mr. Trump, though, is that the Senate is capable of collapsing at the first whiff of political buckshot, without a fare-thee-well to due process.