The iron triangle that has long driven the outcomes of Democratic primaries, especially presidential primaries, re-appeared in South Carolina and re-asserted its dominance on Super Tuesday. This iron triangle is encased in Geritol and does not use either Snapchat or Twitter very much.

What is the iron triangle? Simply put, a majority of Democratic primary voters fall within a demographic pyramid. At the base of this triangle, are minority voters (the aggregate of black, Hispanic, Asian, and multiracial voters) with black voters—and especially black women—casting the largest share of minority primary votes (roughly a quarter of the Democratic primary vote nationally is cast by African-Americans).

On one side of the triangle are highly-educated women, mostly centered in the metropolitan clusters. In most Democratic primaries between 56 percent and 58 percent of the total turnout is female. The force field for this female majority are 40 and older in age, highly-educated professional women, particularly those who live in the suburbs.

The final side is comprised of white ethnics, whose roots are distinctly blue collar. Meanwhile, the benefits of higher education have broadened the range of this voting bloc, expanding it from those with a high school (or less) in education to an ever larger portion of highly-educated professionals who range from the middle class to affluence.

Along both coasts and in the Midwest this white ethnic bloc is heavily White Catholic, with significant pockets of Jewish voters. In the South and the Plains states, white ethnics are a mix of white Catholic, and Mainline Protestants, many of whom are veterans. This white ethnic bloc is varied and difficult to measure, but can range from a quarter to a full third of the Democratic primary electorate in the aggregate (though its local share varies from state to state).

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Within the triangle, many voters have connections to multiple sides.

For instance, there are increasing numbers of both highly-educated, professional minority voters and white ethnics with blue collar roots, whose educational attainments have moved them into higher income brackets.

What’s important to understand is that within the context of Democratic primary voters nationally, somewhere between 60 percent and 65 percent find themselves living somewhere within this triangle.

In a presidential primary if a candidate can corner overwhelming support from two sides of the triangle, they will be at a marked advantage. If a candidate dominates all three sides, then he or she will almost certainly win contest.

On Super Tuesday, Biden cornered all three sides of the iron triangle everywhere except for the two of states that are demographic outliers—Vermont and Utah. This dominance propelled him to victory in ten states and kept him close even in the two states he lost, Colorado and California.

Colorado and California are home to a a growing plurality of what I call pure progressive voters. This bloc has grown from less than a quarter of the Democrats’ primary electorate to about a third of the party’s primary base nationally, over the last decade. In caucus states their share can surge towards a 40 to 45 percent of overall turnout, though in primary states that force-multiplier effect of enthusiasm is much diminished.

This progressive wave is younger and more heavily male than those within the iron triangle and it is stronger among Democratic-leaning independents than it is among rank-and-file Democrats. In terms of education level, it runs the full gamut. The most notable aspect of the pure-progressive bloc is that it is overwhelmingly white. It’s strongest in gentrifying and bohemian urban centers, as well as the small towns and exurbs surrounding university communities.

In a straight-up contest, the voters within the iron triangle will, if united around single candidate, handily defeat the pure progressives.

But primary outcomes tend to be determined by which bloc is united and which is splintered—and whether one candidate has the ability to cross over and build bridges between these two blocs. That was Barack Obama’s great accomplishment: Carrying both minority voters and the pure progressives is what allowed him to upset Hillary Clinton in 2008.

Which brings us to Bernie Sanders.

Sanders has shown the ability to win a large share of Hispanic voters in the Western states (Nevada, Colorado, and California). But this achievement was not replicated back East: In Virginia, Biden carried Latinos, 47 percent to 35 percent, according to exit polls. In Texas Sanders’s performance with Latinos came down somewhere in the middle (which is part of the reason Biden carried Texas).

Texas is worth studying. The exit polls showed Biden narrowly losing the white vote (which was 44 percent of the total) to Sanders, 32-29—but narrowly carrying the non-white vote (which was 56 percent of the total turnout) 37-35.

That non-white margin was due to Biden’s blowout margin of 60-17 with black voters (a 21 percent share of the total), while Sanders carried Latinos by a narrower 45-17 margin (with Bloomberg at a 17 percent among Latinos).

Alternatively in California, exit polls showed Sanders winning the aggregate non-white vote by 45-23, based upon clear wins among Hispanics (55-21over Biden) and Asians (37-16 over Biden).

In California, Biden’s margin with black voters was also much reduced—he only won black voters 38-18, though that can perhaps be explained by 1) the enormous share of early votes cast, which diminished the effects of South Carolina and 2) the heavy ad spending by Michael Bloomberg in California, which won him 20 percent black votes.

California was a nearly perfect fit for Sanders: It allowed him to bank early votes based on his Iowa and New Hampshire momentum; the state has a high percentage of pure progressives; Black voters comprised only 7 percent of the total primary vote; the electorate skewed slightly younger (only 60 percent was over the age of 45); and Michael Bloomber spent heavily in the state and absorbed enough votes to come in third.

And yet, despite all of this, Sanders was only able to beat Biden in California by 8 points. Such is the power of the iron triangle: Even when you’re overmatched and in tough terrain, controlling it can keep you in the game and accumulating delegates.

Some points to consider as the primary race moves forward:

In states where the black voters make up more than 20 percent of the electorate, Sanders will be a distinct underdog.

Where the voters over 45 cast anywhere near two thirds of the total vote, Sanders will be swimming upstream.

Where a majority of a state’s vote is driven by metro clusters (one or more large urban cores surrounded by affluent suburbs) Biden will have a big edge driven by college-educated voters (e.g., Illinois, New York, Rhode Island, Pennsylvania, and New Jersey).

As turnout expands, the pure progressive Sanders will lose, because he has not been able to break into the iron triangle.

It’s worth talking about turnout in some depth, because the reality is very different from the conventional wisdom.

Bernie Sanders has based his entire campaign around the idea that the way to win is by expanding turnout.

There is simply no empirical evidence to support this idea.

Sanders has always done best in caucus states where turnout is lower than in primaries. For example, on Super Tuesday, Sanders lost three states to Biden that he carried by wide margins in 2016 (Maine, Minnesota, and Oklahoma). Why? Because each of those states were caucuses when he carried them four years ago. Once they were transformed into high-turnout primaries, Sanders lost them.

In addition, the states where Biden swept—such as Virginia and North Carolina—saw large turnout increases from 2016. Virginia went from 782,000 in 2016 to 1.32 million this year; North Carolina swelled from 1.14 million in 2016 to 1.312 million. Biden’s upset victory in Texas also benefited from a turnout increase: from 1.44 million in 2016 to 1.97 million last week.

It is true that Sanders has the potential to expand turnout among the segment of the primary electorate that is under 30. The problem is that those voters comprise just under a fifth of the Democratic primary electorate. And even that potential is merely theoretical for Sanders: He has not actually demonstrated the ability to match Barack Obama as turnout engine for younger voters. (Obama took the under-30 share from 13 percent of the total vote to about 20 percent of the total vote in the 2008 general election).

It’s just math: When your base is a third of the primary vote—which translates into less than 15 percent of the vote in the general election—then your turnout prowess is destined to have a marginal impact. Which can, in theory, be significant, but is rarely dispositive.

I am not saying Sanders has no chance to beat Biden. Sanders is nothing if not tenacious.

But I am saying that Sanders can’t defeat Biden unless and until he can deeply raid at least two of the three sides encasing this iron triangle: minority voters (especially black voters), highly-educated women from the metro clusters, or white ethnic voters.

So let’s project how this presidential primary contest is likely to play out from here.

In the March 10 primaries, Sanders will be favored in Idaho, North Dakota, and Washington state (123 delegates at stake), while Biden would be favored in Mississippi and Missouri (104 delegates). This leaves Michigan’s 125 delegates as the fulcrum for determining the perceptual and actual winner of the day.

Michigan—which Sanders narrowly won on the trade issue in 2016—is Sanders’s best chance to blunt Biden’s clear and unmistakable momentum from South Carolina’s Super Tuesday. In 2016, Sanders carried Michigan’s white ethnics and it enabled him to pull off a narrow upset in Michigan.

Then, on March 17, Arizona, Florida, Illinois, and Ohio will allocate 577 delegates. And a week after that comes Georgia with its 105 delegates.

In terms of age, race, and regionalism (the dominance of metro clusters) these states will reflect the tensile strength of the iron triangle. To avoid an insurmountable lead for Biden in terms of both delegates and aggregate votes (from all of the Super Tuesday states combined—and with 93 percent of California’s vote reporting—Biden has so far received 824,000 more votes than Sanders), Sanders needs to:

Win Michigan.

Avoid getting blown out in the March 17 primaries.

Hold his own in the Georgia primary on March 24.

Any one of these tasks is a heavy lift for Sanders based on his electoral coalition. Hitting the trifecta? Let’s just say that if Sanders is trailing Biden by 250 delegates on March 25 he will have the look of a sure loser.

Sanders does have an opportunity to rebound—at least perceptually—on April 4 in Alaska, Hawaii, and Wyoming. But those states account for 53 delegates and on the same day Louisiana will apportion 54 delegates in a race that is almost certain to go overwhelmingly to Biden.

The truth is that Sanders’s best chance after Michigan won’t come until April 7, when Wisconsin votes alone to allocate 84 delegates. In 2016, Sanders swept Wisconsin.

After Wisconsin there is a lull of three full weeks before the next primaries on April 28: New York, Pennsylvania, Connecticut, Maryland, Rhode Island, and Delaware (633 total delegates). This will be the last big delegate haul of the 2020 primary calendar.

And these five states are the poster children for the electoral power of the iron triangle.

They are diverse on race (more heavily black than Hispanic), with large metro clusters, large white Catholic and Jewish populations, and the base vote in these states is older (along gender, race, religion and region).

I am not prepared to project with certainly whether Biden or Sanders will win the nomination (although the remaining primary calendar clearly favors Biden). But I can state unequivocally that the winner will be the candidate who masters the iron triangle down the stretch.