It made headlines when former Secretary of State Colin Powell said in an interview Sunday that he would vote for Democrat Joe Biden in November and “cannot, in any way, support President Trump.” But it wasn’t really news. Though Powell is a Republican, he has been endorsing Democratic presidential candidates for years: He backed Barack Obama in 2008 and 2012, and Hillary Clinton in 2016. Four years ago he described Trump as a “national disgrace” and his opinion hasn’t improved. Trump “lies all the time,” said Powell, and “has not been an effective president.”

Powell opposes Trump? Tell us something we don’t know.

But Powell isn’t alone. Former President George W. Bush won’t vote for Trump, either, the New York Times reported. Neither will Senator Mitt Romney. Nor Cindy McCain, the widow of Senator John McCain. The last two Republican House Speakers, John Boehner and Paul Ryan, decline to say whether they will vote for Trump. Likewise former Secretary of State Condoleezza Rice and Trump’s former chief of staff, retired Marine Gen. John Kelly. Senator Lisa Murkowski, an Alaska Republican, tells reporters she is “struggling” with whether to reelect the president. Representative Francis Rooney, a Florida Republican and former ambassador to the Vatican under Bush — and high-dollar donor to past Republican candidates — says he is thinking seriously about supporting Biden, since Trump is “driving us all crazy.”

Last month, a new organization called Republican Voters Against Trump launched a $10 million ad campaign aimed at Republican-leaning voters in battleground states. It features conservative or GOP voters, many of whom voted for Trump in 2016 but cannot bring themselves to do so again. “I’d vote for a tuna fish sandwich before I’d vote for Trump again,” says Jack Spielman of Detroit.

Spielman may not change many minds, but the same probably can’t be said of Jim Mattis, the acclaimed four-star Marine general who resigned in December after two years as secretary of defense. Mattis had for months refused to publicly criticize the president. He broke his silence last week, condemning Trump for deliberately inflaming discord and turning Americans against each other.

“Donald Trump is the first president in my lifetime who does not try to unite the American people — does not even pretend to try,” Mattis declared in The Atlantic. “Instead he tries to divide us… . We are witnessing the consequences of three years without mature leadership.”

It was an extraordinary denunciation, coming as it did from one of the nation’s foremost warrior-statesmen. It was as if George C. Marshall had turned against Harry Truman in 1948. What Mattis said publicly is what many Republican officeholders say privately; perhaps his words will induce more of them to come out into the open.

To date, Trump’s popularity among the Republican faithful has been impregnable, and there is no sign that the GOP base is wavering in its loyalty. Could that change?

If history is any guide, the near-certain answer is no. Only once has the Republican Party denied an incumbent president its nomination for another term: In 1884, it abandoned Chester Arthur, angry with him for having turned against the corrupt patronage system. By all the conventional laws of politics, Trump will be renominated at the GOP convention this summer.

But what if Trump’s unpopularity (57 percent in the latest CNN poll) continues to climb? What if Biden’s 14-point lead over Trump continues to widen? What if more Republican and conservative leaders, their spines stiffened by Mattis’s example, come out publicly against the president? What if GOP candidates come to fear that Trump will drag them down to defeat? What if Senate Republican leader Mitch McConnell concludes that renominating Trump will imperil his Senate majority? What if it becomes clear that the American electorate will never reelect a president who was impeached?

To repeat, Trump’s GOP wall of support seems solid. But when cracks appear in walls, they make other cracks more likely. The Soviet Union seemed unassailable too — right up until it became untenable. “If Republicans ever turn on Trump, it’ll happen all at once,” political scientist Lee Drutman predicts.

Republicans should never have embraced someone so freakish and ill-suited for the presidency. The mistake they made in nominating Trump in 2016 cannot be undone. But it isn’t too late to decide against repeating that mistake in 2020, and to turn instead to a candidate who can make America, and the GOP, normal again.

(Jeff Jacoby is a columnist for The Boston Globe).