Call it the socialist scenario — the risk that Bernie Sanders and Elizabeth Warren could combine forces to defeat Joseph Biden in the Democratic primary.

The RealClearPolitics polling averages have Biden leading Sanders and Warren nationally and in the early-voting states of Iowa and New Hampshire. These same polls, though, show the “not Biden” vote represented by Senators Sanders and Warren to be larger than the level of support for Vice President Biden.

If that vote were combined rather than split, the socialist scenario suggests, it could result in a Democratic presidential nominee who is either openly socialist, like Senator Sanders, or an ideological ally of Mr. Sanders, like Mrs. Warren, who says she is a capitalist but who is campaigning with a call for an annual wealth tax and for what she calls “big, structural change.”

The possibility is generating concern from Americans who are more cautious about “big, structural change.” The concern is heightened because Mr. Biden is old enough that he can seem vulnerable rather than inevitable.

As is often the case with socialism, however, the fantasy is some distance from reality. The primary campaigns of the previous presidential cycle are familiar precedents and somewhat reassuring ones, at least for those who aren’t enthusiasts of either Mr. Sanders or Mrs. Warren.

On the Republican side in 2016, the fact that there were more “not Trump” voters than Trump voters wasn’t ever enough to stop Mr. Trump; the “not Trump” vote was fractured among a variety of candidates such as Governor Jeb Bush and Senators Marco Rubio and Ted Cruz, just as the “not Biden” vote is currently split. The way these contests are structured, with a large field of candidates, you don’t really need a majority, just a strong plurality, at least until the nominating convention.

On the Democratic side in 2016, Mr. Sanders drew a lot of votes in 2016, too — 43%, by one incomplete count cited by Wikipedia. Yet in 2016, the socialist opposition wasn’t enough to defeat the establishment front-runner, who in 2016 was not Vice President Biden but Secretary of State Clinton. It’s not clear that the Democratic primary electorate has moved so far left since 2016 that re-running a similar experiment with Mrs. Sanders in 2020 will produce appreciably different results than it did in 2016.

It is hard to imagine Mr. Sanders endorsing Mrs. Warren until and unless Mrs. Warren defeats him consistently and by a significant margin in several early states. Similarly, it’s difficult to see what Mrs. Warren would get from throwing her support to Mr. Sanders.

Even if Mrs. Warren were promised the vice presidency on a Sanders-Warren ticket, Democratic voters who are excited by her might be dismayed to see a woman of color — if you credit Warren’s claim to Native American ancestry — stepping aside to play a merely supporting role. Mrs. Warren herself might resist the idea and prefer to persist in the quest to make history as the first woman president.

Even if Mr. Sanders or Mrs. Warren somehow managed to win the Democratic nomination, they’d have to defeat President Trump actually to make it to the presidency. That wouldn’t be easy. An incumbent president running for reelection hasn’t been defeated since Bill Clinton beat George H.W. Bush in 1992. Trump will seize the opportunity to define his opponents: “crazy Bernie,” “Pocahontas.”

Even if Senators Sanders or Warren somehow did manage to defeat Mr. Trump and win election as president of America, the former Burlington, Vermont, mayor or the former Harvard law professor would have a hard time fully implementing a socialist or “big structural change” program in a system that requires 60 Senate votes for a lot of legislation.

President Trump ran promising to build a border wall and repeal ObamaCare. He has done neither, despite having Republican control of both the House of Representatives and the Senate for his first two years. Not that Mr. Trump hasn’t accomplished anything at all. He has done a lot. Senators Sanders or Warren would probably do a lot, too, if either one became president. It’s just that Washington has a way of stalling ambitious plans.

That reality may, paradoxically, account for some of the support out there for Senators Warren and Sanders. If voters really thought these candidates were going to implement their programs fully, the voters might be more hesitant. By the time Sanders-style socialism or Mrs. Warren’s “big, structural change” finally make it through the Washington compromise meat-grinder, they might eventually wind up looking less revolutionary and more incremental.

It’s not a risk I’m eager to take. It may not be a risk that the majority of the Democratic Party is willing to take. Trump sympathizers may be consoled that some of the same factors — Congress, the “deep state” bureaucracy, entrenched special interests — that have hampered President Trump could provide some brakes or guardrails if the socialist scenario comes to pass and Mr. Sanders or Mrs. Warren wind up in the Oval Office.

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Image: Campaign poster of Socialist Party of America, 1904; via Wikipedia / Ebay