The debate was about Elizabeth Warren because the election is now about Elizabeth Warren

(AP Photo/Craig Ruttle)

Democratic presidential candidate U.S. Sen. Elizabeth Warren takes the stage before addressing supporters at a rally, Monday, Sept. 16, 2019, in New York.

• Sen. Elizabeth Warren is the frontrunner, and along with the advantages that brings she was also the object of some heat tonight in Ohio.
• Five candidates fighting for their spot in this race have a huge vulnerability to Elizabeth Warren.
• Sen. Amy Klobuchar, Mayor Pete Buttigieg, and former Rep. Beto O'Rourke are each highly vulnerable to Warren and went on offense tonight.
• Sen. Cory Booker and former Housing and Urban Development Secretary Julian Castro are also highly vulnerable, but they went in a different direction.

The fourth Democratic presidential debate was a baptism by fire for the party's new frontrunner Sen. Elizabeth Warren. Previous debates have been opportunities for contenders to land punches on former Vice President Joe Biden, then a clear and unrivaled frontrunner, or a chance for more centrist candidates to draw major policy distinctions between themselves and Sen. Bernie Sanders, the most recognizable and popular stalwart of the party's left flank.

This debate was different. This debate was about Warren.

In part, that's because she's the frontrunner. But it's actually a little more significant than that: right now, half of the people on stage will have their political futures determined by the success or the failure of Warren. Here's why.

Insider has been conducting a recurring SurveyMonkey Audience poll since December focused not on the topline numbers, but instead on the overlapping pools of support.

Read more: How the INSIDER 2020 Democratic primary tracker works

Business Insider

Data as of late September.

We ask Democratic respondents who they would be satisfied with as nominee. Warren has been steadily rising on that front: At the beginning of the summer, half of those familiar with Warren were satisfied with her as nominee.

Concentrating at the four polls since September, Warren's satisfaction is up to 66%, which is now vastly better than the 57% of those aware of Biden who'd be satisfied with his nomination.

Warren is eating these candidates' lunch.

Here's the larger picture, though. Among people who'd be satisfied with Sen. Cory Booker, Mayor Pete Buttigieg, former Rep. Beto O'Rourke, Sen. Amy Klobuchar, and former HUD Secretary Julian Castro, they disproportionately love Sen. Warren. In the four polls since September:

• 82% of people satisfied with Buttigieg are also satisfied with Warren
• 85% of people satisfied with Booker are also satisfied with Warren
• 79% of people satisfied with O'Rourke are also satisfied with Warren
• 87% of people satisfied with Castro are also satisfied with Warren
• 83% of people satisfied with Klobuchar are also satisfied with Warren

Reminder, the overall satisfaction rate of people who are familiar with Warren is 66%. That's a massive, defining vulnerability for each of these individuals. That's ironclad math: you cannot win an election when four fifths of your fanbase is content with the victory of the frontrunner.

This is a one-way vulnerability. Warren doesn't have to worry about these candidates, yet.

More to the point, the feeling is not mutual. Looking at the 867 Democrats satisfied with Warren as nominee across the four polls:

• 44% are also satisfied with Buttigieg
• 35% are also satisfied with Booker
• 33% are also satisfied with O'Rourke
• 22% are also satisfied with Castro
• 19% are also satisfied with Klobuchar

The math of the debate gauntlet is merciless for middle-tier contenders, as only two-thirds of the candidates on stage in Ohio have punched their ticket to the November show. The party - eager to winnow the field - will continue pulling the ladder up until the debates more resemble a rock band than a casino heist in terms of participants. This is ending, and sooner rather than later.

This debate saw candidates pick their sides.

Klobuchar, O'Rourke and Buttigieg went on offense: realizing that they still have a shot at seizing the lane for the moderate wing of the party, they went after Warren and - at least in the case of Buttigieg - arguably landed some pretty significant blows on the frontrunner.

Booker and Castro, despite their overwhelming vulnerability to Warren, did not at any point directly attack her. It's possible that those candidates are betting on Warren's star falling. It's also possible that given their close relationships with Warren - Booker as a fellow northeast Senate colleague, Castro as another former Obama administration official - and possible constituent synergies, they're reminded that a presidential nominee necessitates the presences of a Vice Presidential nominee.

For one reason or another, the five candidates who constitute the bulk of the middle tier, their political career either goes through or alongside Warren.

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