Evidence since 2021 shows that an insider vs. outsider / establishment vs. anti‑establishment cleavage has become a persistent, cross‑cutting theme in U.S. politics, alongside the traditional left–right divide, matching Sacks’s forecast.

1. Populism and anti‑elite rhetoric are now central to U.S. politics. Political science and reference works describe contemporary right‑wing populism (Trumpism) as built around anti‑elitist, anti‑Establishment appeals, explicitly framing politics as “the people” versus corrupt elites rather than just standard ideology. (en.wikipedia.org) Analyses of Trump’s rhetoric emphasize that his core narrative is a crisis caused by a corrupt political establishment and that he offers himself as a “high‑risk outsider candidate” to overthrow it. (en.wikipedia.org) This outsider‑vs‑establishment framing has remained central through and beyond the 2024 campaign, not just in 2016.

2. The idea that both parties are a single corrupt insider ‘uniparty’ has gone mainstream. The term uniparty—claiming that Democrats and Republicans operate as one insider cartel—has been widely used in the 2020s by nationally prominent figures like Steve Bannon, Rep. Marjorie Taylor Greene, and especially Robert F. Kennedy Jr. in his 2024 independent presidential campaign, where he denounced the two parties as a “uniparty.” (en.wikipedia.org) This is exactly an “insiders vs. outsiders” frame: both major parties are cast as self‑dealing insiders, while various populist or independent figures claim to represent excluded outsiders.

3. Candidates across the spectrum deliberately brand themselves as ‘outsiders’ against party establishments. A 2025 Guardian profile of Michigan Senate candidate Abdul El‑Sayed describes him running an explicitly populist, anti‑establishment campaign and repeatedly labels him “a true outsider,” noting that even one of his more moderate Democratic opponents is also styling herself as an outsider despite using establishment consultants. (theguardian.com) Commentary on other would‑be candidates (e.g., pieces floating Stephen A. Smith or other non‑politicians for the presidency) likewise centers on their appeal as political outsiders who can challenge an entrenched establishment and disillusioning party leadership. (newyorker.com) This shows the insider/outsider language has become a standard, recurring way of marketing and analyzing candidates.

4. Analysts explicitly describe new fault lines that map onto ‘insider vs. outsider’. Commentators and scholars have argued for years that U.S. politics is realigning from a simple liberal‑vs‑conservative split toward a divide between populists and establishment neoliberals or technocrats—an axis that cuts across both parties. (thefederalist.com) Research on populism notes that populist movements systematically dramatize an insider–outsider conflict—“the people” versus “corrupt elites” and other alleged outsiders—and that this pattern has become a core, global theme in 21st‑century democratic politics, including the United States. (populismstudies.org) This is substantively the same cleavage Sacks labeled “insider versus outsider.”

5. Left–right polarization remains very strong, but Sacks only predicted ‘not just’ left vs. right. Empirical work still finds that partisan and ideological polarization (Democrat vs. Republican, liberal vs. conservative) has grown sharply and remains the primary organizing structure of mass attitudes. (arxiv.org) However, Sacks’s claim was more modest: that in the post‑Trump era it’s not just about left and right anymore and that insider vs. outsider would be a major recurring theme, not that it would replace left–right entirely. Given the entrenched prominence of populist anti‑establishment rhetoric, the spread of ‘uniparty’ and elite‑vs‑people narratives, and the routine branding of candidates in insider/outsider terms across both parties and among independents, that part of his prediction has clearly materialized.

Because the insider–outsider / establishment–anti‑establishment framing has, by 2025, become a pervasive and widely discussed lens for understanding U.S. politics—while Sacks did not require it to supplant left–right altogether—the prediction is best judged as right.