There will be necessary internecine fights, and they boil down to loyalty tests on particular positions demanded by the vanguard. Consider the debate within the party on Senator Bernie Sanders’s “Medicare for All” bill, which most (though not all) 2020 contenders rushed to attach themselves to. To fail to sign on to that legislation is to open oneself to criticism, even abuse, although it’s less a piece of legislation than a goal.

Forget about who’s right and wrong in these debates. Time will sort that out. My point is that they tend to consume a party experiencing a shift. The Democratic Party, because it is an amalgam of interest groups in a way the Republican Party is not, has always had a tendency to elevate the candidate who can check the most boxes. The current internal dynamics exacerbate that. It’s also worth remembering that no one besides party activists cares.

So when the party’s leaders tussle over this or that policy, they also need to take a step back, to see the direction the country — the West itself — is heading, and take a stand on it. This isn’t just a matter of high-minded idealism; it’s what separates great politicians from merely good ones.

History tells us that the transformative politicians, the ones who can change the country’s direction and will really matter in the history books, are the ones who can do both. I think there have been four of them in the past century: Franklin Roosevelt, Ronald Reagan, Bill Clinton and Barack Obama.

Why Roosevelt and Reagan should be obvious. I know some would dispute my choice of Mr. Clinton, but he rescued a party that had lost three presidential elections in a row and was being read last rites by some pundits in 1991 (the extent to which he changed the country’s fundamental direction is debatable). Mr. Obama made history and redrew the electoral map. All four were able to speak both to their base and beyond it by identifying the challenge of the moment and persuading majorities that they had some answers.

The future of the Western democratic project is the fundamental issue of our era. It’s under attack from Vladimir Putin and Steve Bannon and many people in between (and to the extent that he backs Mr. Bannon’s purge of the Republican Party, from the president himself; think about that).

Democrats can’t duck this question and expect the broader electorate to see them as prepared to lead. To his credit, Mr. Sanders did talk a bit about all this in a foreign-policy speech in late September at the same Missouri college where Winston Churchill gave his Iron Curtain speech, noting an “international order” that is “under great strain.”

The Democrats were the party that created this order after World War II. They must now be the party that fixes and saves it.