This much is certain: America’s political parties are in transition. What is uncertain is what they are evolving into, and whether the changes now underway are provisional or permanent.

Here is an oversimplification: The Republicans are becoming a Donald Trump party, and the Democrats are becoming a Bernie Sanders party. There are plenty of reasons to dispute that, and I might agree with most if not all of them. But it is a generalization, and it surely is what each of the parties says about the other party.

At the same time, there are curious subterranean streams in each party. A rump of the Republican Party remains skeptical if not contemptuous of Trump. The former president calls them RINOs, an acronym for “Republicans in Name Only,” which is not quite (or even remotely) accurate, because they represent what the party was only a few years ago.

There remains a rump of the Democratic Party that is skeptical of the leftward lurch of the most prominent members of the party, especially Sen. Sanders and Rep. Alexandria Ocasio-Cortez of New York. Democratic leaders treat those skeptics as recalcitrants, but they are making a serious error by believing that that group consists only of the senators from Arizona and West Virginia.

The movement toward the extremes is an aberration in American politics that doesn’t even have an antecedent in the runup to the Civil War, a reference that today is thrown around the American commentariat with promiscuous abandonment. In that long-ago, fraught era, the Republicans treated the gathering crisis as one of the endurance of the Union rather than what it was: a cultural conflict over the endurance of slavery. The Democrats treated it as a matter best attended to with the doctrine of its leading figure, Stephen A. Douglas, though his “popular sovereignty” in the end produced only chaos and bloodshed.

Where the current dysfunction and dystopia go is beyond the ken of any analyst, though Sen. Joe Manchin, the West Virginian who has caused so much agita among his fellow Democrats, and Sen. Lisa Murkowski, the Alaska Republican who has caused so much aggravation in Trump’s camp, did an interesting thing the other day. They said they would endorse each other.

Let’s lay that out for what it is: an unusual but perhaps revealing development. A Democratic senator endorsed his Republican colleague, and a Republican senator said she would back a Democrat. This does not happen every day, though Manchin did endorse Sen. Susan Collins of Maine — the Republican lawmaker whom Democrats nationwide rallied, unsuccessfully, to defeat — in her 2020 reelection battle.

Party regulars hate this sort of thing, and in a way congressional tradition militates against it (though according to a new book by Irwin Gellman, John F. Kennedy once quietly gave Richard Nixon a $1,000 contribution for his 1950 Senate race). American parties have few elements of discipline, but one of them is to stick with their elected incumbents. The Senate itself has even fewer elements of discipline, but one of them is that members do not get involved in colleagues’ races. That is why Senate Majority Leader Bill Frist’s 2004 campaign visit to South Dakota to defeat his majority-leader predecessor, Democrat Tom Daschle, created an uproar.

But here we have one dynasty aiding another. Manchin’s father and grandfather were both mayor of his hometown of Farmington, West Virginia, and his uncle was a member of the state’s House of Delegates and its secretary of state. Murkowski’s father was a senator, governor and state economic-development commissioner. Disrupters — that’s what Trump is, and what Democratic progressives seek to be — have no taste for dynasties, though the progeny of Trump may test that.

This mutual cross-endorsement — a double toe loop of politics — is a political oddity. But mainstream politicians have had a grudging openness to embracing their rivals as a way of healing the political body politic.

Sen. John Kerry of Massachusetts, the 2004 Democratic presidential nominee, hoped to lure Republican Sen. John McCain of Arizona onto his ticket. McCain, the GOP nominee four years later, gave serious thought to inviting Sen. Joseph I. Lieberman of Connecticut, the Democrats’ 2000 vice-presidential nominee, to take the same role on the Republican ticket; he had earlier endorsed Lieberman for the Democratic nomination in 2008, which might have made him his general-election opponent. Biden said in his 2020 presidential campaign that he was open to considering a Republican running mate.

None of that happened. The American political scene might be different if it did.

Now Biden is president during a period of unusual uncertainty and peril, democracy itself in the balance — a notion shared by both Republicans and Democrats, though for vastly different reasons and with vastly different views of how to address the crisis. Biden said last month that Kamala Harris would be his running mate again should he seek a second term. Yet there are some voices who believe that he, or another Democratic nominee if the president doesn’t run for reelection, should consider a Republican running mate, someone along the lines of Sen. Mitt Romney of Utah, who was the GOP presidential nominee in 2012. (The notion raised a chorus of dissent among Democrats, including the argument that one precedent, Abraham Lincoln’s selection of the Democrat Andrew Johnson in 1864, was a disaster.)

Nor is the United States likely to reach for the 1931 British example, when, amid the Great Depression, political leaders assembled what was known as the National Government, drawing together Conservatives, some Liberals and a handful of Labour members. Its advantage: It provided political stability at a time of great social and economic instability and kept political extremism at bay. Its disadvantage: It opened wounds that festered for years — and in the end failed because it was too orthodox in economic matters, it didn’t address social collapse in the shipbuilding, coal and steel sectors of industrial Britain, and it was all but blind to the Nazi threat in Europe.

Loads of Republicans and Democrats at the extremes would feel they were filtered out for a centrist coalition in that eventuality. But small gestures of inter-party outreach might be refreshing. What Democrat isn’t quietly rooting for Murkowski? What Republican isn’t covertly rooting for Manchin?

(David M. Shribman is the former executive editor of the Pittsburgh Post-Gazette.)