Both Democrats and Republicans may be playing to their electoral base as they tussle over a recount in Wisconsin. But Donald Trump's actions are the ones we need to fear.

Sunday's conspiracy-driven rant from Mr. Trump that an election he actually won was rigged against him chills the hearts of those who fear the worst from his presidency but held hope in recent days for something better.

Read more: Trump, with no evidence, claims millions voted illegally

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It confirms that this man, when challenged, lashes out in the most ridiculous and dangerous ways. God only knows what may happen when that challenge comes, not from a domestic opponent, but from another president, from someone willing to meet threat with force.

Mr. Trump faces the uncomfortable truth that, although he won fair and square in the electoral college, he is on track to lose the popular count to Ms. Clinton by upwards of 2.5 million votes.

Jill Stein, the Green Party presidential candidate, is raising the money needed to force a recount in key swing states that Mr. Trump won by very narrow margins. The Clinton camp announced on the weekend that it is throwing its support behind the recount effort in Wisconsin, saying the Democratic presidential nominee owed it to her supporters, even though she did not expect anything would change.

Mr. Trump correctly pointed out earlier this month that, if the election were decided on the popular vote rather than at the electoral college, he would have campaigned in populous states that traditionally vote Democratic, such as California, New York and Illinois, to drive up his numbers. That is a sound argument.

On Sunday, he went in a very different direction, tweeting: "In addition to winning the Electoral College in a landslide, I won the popular vote if you deduct the millions of people who voted illegally."

This is a baseless claim. Although his surrogates on Monday pointed to studies showing ineligible voters were on the rolls in past elections, there is zero evidence millions of people voted fraudulently to support Ms. Clinton. Mr. Trump's tweet is the stuff of fantasy, or paranoia.

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The president-elect's tweet might have been based on an earlier report in Infowars, an extreme right-wing website that caters to the tinfoil-hat crowd, declaring that "three million votes in the U.S. presidential election were cast by illegal aliens." We can only hope they were referring to undocumented immigrants.

While both sides are obviously playing to their core partisan supporters, Mr. Trump's reaction is particularly disheartening. Last week, he showed encouraging signs of moderation and mental balance. We learned he is considering appointing Mitt Romney, the Republican presidential candidate in 2012, who has been one of his harshest critics, as secretary of state. And in a wide-ranging Q&A with staff of The New York Times, he announced his administration would not pursue a criminal investigation of Ms. Clinton and that he was open to the notion that climate change might be real and not a Chinese hoax.

But if those actions reflected Mr. Trump on a good day, his tweet-rage on Sunday showed him at his worst: angry, narcissistic and prone to embrace ludicrous conspiracy theories. Claiming millions of illegal voters tried to rig the election for Hillary Clinton is up there with accusing Senator Ted Cruz's father of being involved in John F. Kennedy's assassination.

To further dampen any whiff of optimism, Mr. Trump's campaign manager, Kellyanne Conway, went on television on Sunday declaring that if the president-elect picked Mr. Romney, his supporters would feel "betrayed," which suggests chaotic, faction-ridden infighting over at Trump Tower.

Mr. Trump should have laughed off the recount in Wisconsin as a quixotic waste of time from a defeated opponent. Instead, he is treating the matter as a personal affront. How, then, will he react if tensions rise in the South China sea, the Baltics or the Middle East?

It is as though we are all trapped in an abusive relationship with the next U.S. president.