The United Kingdom’s 2016 referendum to leave the European Union — Brexit — triggered a sweeping and often counter-intuitive realignment inside the UK Conservative Party.  A new study co-authored by Ian Shapiro, Sterling Professor of Political Science and Global Affairs, and University of Arizona professor Christian Cox offers empirical evidence that the Tories’ shifts in response to the referendum were driven less by ideology than by political survival instincts — a finding with relevance to the present political divisions in the U.S.

In an interview, Shapiro explained that the research is “a case study that shows how single-issue politics weakens political parties and empowers fringe groups with intense preferences.” He noted that this finding runs counter to the democratic tenet of voter representation.

The research, published in PLOS ONE, shows that Conservative Members of Parliament (MPs) who resisted switching to a pro-Brexit position faced significantly higher political risks than those who aligned themselves with the referendum result, particularly in “safe seats,” where internal party threats mattered more than general elections.

The authors compiled a novel dataset tracking every Tory MP’s Brexit position from 2015 to 2022. They found that MPs who declined to embrace Brexit after 2016 were disproportionately likely to lose their positions through defeat, deselection, or standing down, while those who switched toward Brexit tended to remain in office. The data show that by 2018 — just two years after the referendum — 262 Conservative MPs publicly supported Brexit while only 124 supported remaining in the EU — a dramatic reversal from 2015 when most Tories were pro-EU.

According to Shapiro, the Conservative Party’s leadership misjudged the dynamics of referendums. Instead of a national vote neutralizing internal conflict by reaffirming Britain’s EU membership, he said, “the intense single-issue Brexiters were better organized and more highly motivated,” leading to the unexpected Brexit victory.

A significant finding from the study is the identification of “entryism” — the threat of internal party challengers — as the main driver of Conservative MPs’ ideological shifts. MPs in safe Conservative constituencies were far more likely to switch to a pro-Brexit stance, not because their personal views changed but that they feared deselection by hardline activists.

Shapiro compared this finding to U.S. primary challenges in solidly partisan districts, where politicians with safe seats may support more extreme views. “The extremes of the party become more and more powerful because the only elections that matter are the primaries,” he said. “So, there’s an analogous dynamic in Britain, where the propensity to switch is much stronger in safe seats than in competitive seats.”

The paper also documents the consequences. In the 2017 and 2019 elections, a large share of the Conservative MPs who stood firm in opposing Brexit were removed, either pushed out by party leadership or defeated by pro-Brexit Conservatives or Liberal Democrats (a centrist third party). By 2019, only 7 of the original 74 anti-Brexit Conservative MPs remained, all of whom had switched to back Brexit.

While switching kept many MPs in office, it left the Conservative Party internally fractured and politically volatile. The study notes that Boris Johnson’s apparent firm control of his party in 2019 — when he removed 21 anti-Brexit MPs — was not a sign of unified ideology but rather “MPs running scared of the possibility of entriest challenges.”

Initially an effort to resolve divisions in Britain and within the Conservative Party, Brexit ultimately has done the opposite, Shapiro said — a rather predictable outcome.

“The use of referendums to avoid confronting fractures within parties almost never works,” he said, noting similar dynamics in the 1975 EU referendum under Harold Wilson. “[Brexit] is a case where an attempt to avoid dealing with a party conflict ended up making that conflict far worse.”