Much has been written about the potential damage Brexit will do to the UK economy, the British union (see related blog) and the future of the EU itself. Almost nothing has been said about the negative consequences of Brexit for the two major political parties that have governed Britain for more than a century.
Yet the past four years have seen a dramatic collapse in internal party cohesion in both the Labour and Conservative parties, prompted almost entirely by deeply-held differences of opinion on the Brexit issue. The numerous defections, splinter groups and caucus revolts that ensued – to say nothing of the emergence of fringe parties whose sole reason for being was the promotion or rejection of Brexit — appear to have taken most party stalwarts and outside observers by surprise, when in fact they could easily have been predicted. These events offer further proof that political parties, no matter how well entrenched or stable, can be unravelled by issues that cut across the left-right spectrum on which parties are traditionally placed.
Canadians know this only too well. Here federalism has served as such an issue, representing a cross-cutting cleavage on the centralization-decentralization continuum identified by Livingston. The importance of federalism as an alternative cleavage is easily seen by examining the historic positions of the three main political parties of the post-war era. The Liberal Party of Canada has traditionally favoured a strong centralist approach to federalism, while both the Progressive Conservative and New Democratic parties have supported a far more decentralist vision of federalism.
More recently, the Liberal Party itself was nearly destroyed by intraparty strife along the federalist axis, prompted by the Mulroney Progressive Conservative government’s introduction of the Meech Lake Accord in 1985. That Accord was widely viewed as antithetical to the Liberal approach to federalism. Not surprisingly it was supported by the NDP, and roundly rejected by the vast majority of individual Liberals as well as the Liberal caucus. When the Liberal leader of the day, John Turner, announced that he would support the deal and require his caucus to do the same, a period of fierce internal party strife ensued that rocked the ‘natural governing party’ to its core. The debate saw both social (left-of-centre) and business (right-of centre) liberals – who otherwise would have found themselves on opposite sides of internal party debates over left/right issues such as social welfare or trade — join forces against the deal and their party leadership. In the end Meech Lake undid Turner’s leadership. But the party did not collapse. Instead, it saw Jean Chretien, the only strong opponent of Meech Lake in a crowded field of candidates led by Paul Martin Jr, win the leadership of the Liberal Party in 1990 on the strength of his opposition to Meech Lake and support for a centralist vision of federalism. 
Echoes of the federalism cleavage in the Liberal Party could still be seen years later in the internal struggles between Chretien and Martin supporters over the Clarity Act, and Ignatieff and Dion supporters in the 2006 leadership race. It could also be seen in the decision of a young Justin Trudeau to back leadership candidate Gerard Kennedy in that race, on the basis of Kennedy’s outright rejection of the ‘Quebec nation’ resolution introduced in the House of Commons by then Conservative prime minister Stephen Harper, a motion atypically supported by other Liberal leadership candidates including Dion and Ignatieff. 
As outlined in detail elsewhere, both Britain’s Conservative and Labour parties have undergone periods of intraparty strife over a similar cross-cutting issue, namely, the country’s membership in the European Union. Much like the federalist-decentralist conflict, the issue is one of greater or less political integration with the EU. Whether to join at all, and then how closely the UK should be aligned, has been an ongoing dilemma for both parties for decades, regardless of whether they found themselves in government or on the opposition benches. Indeed, when Britain finally joined the EU precursor EEC in 1973, the Heath Conservative cabinet and caucus were so badly split that Heath would not have succeeded without the help of 20 Labour MPs, who broke ranks with their leader, Harold Wilson, to support the Conservatives, while another 20 Labour MPs abstained.
Nor was David Cameron’s ill-fated referendum on EU membership in 2016 the first such consultation with British citizens. In 1974 Labour formed the government under Wilson, who then changed his position and decided to support EEC membership, a move which immediately provoked a caucus revolt, a special one-day party convention and mass resignations from the party executive before it was eventually decided to hold a popular referendum on whether to remain or withdraw from the EU. The pro and con forces during the referendum campaign cut across party lines, while the pro-Remain Labour government saw 7 of its 23 cabinet ministers support Withdrawal. The Remain side nevertheless won the popular vote by a margin of 2 to 1, but party divisions remained. By 1981 four of the seven Labour ministers had left the party entirely to form the breakaway Social Democratic Party. Similarly the Thatcher and Major Conservative governments that followed also faced numerous internal conflicts over how much further the UK should integrate with the EU, culminating in John Major’s about-face, declining to join the euro zone after having argued in favour. As was soon evident, this reversal was a direct result of a caucus revolt which eventually saw him expel 8 MPs and forced him to govern for the remainder of his term with a minority. In yet another example of déjà vu, the Blair Labour government, arguably the most pro-EU in the UK’s history, still failed to move the country into the euro zone. With the public ready to go along and economic indicators favouring the move, Blair nevertheless abandoned his efforts due to internal caucus strife, and in particular to the strenuous objections of his heir apparent and chancellor, Gordon Brown. By early 2002, the Labour caucus had actually organized itself into three formal groups: “Labour Against the Euro”, “Labour Movement for Euro” and the “Labour Euro-Safeguards Campaign,” each of which had 30 or more MPs and well-known leadership. As a result, by the time David Cameron called the 2016 referendum, it should have come as no surprise that a significant motivation for his decision was to quell once and for all the ongoing opposition to EU membership within his Conservative caucus.
Nor should the subsequent three-year drama that has unfolded over Brexit in the Conservative and Labour parties be seen as an aberration. Yet where it has been mentioned at all, the meltdown of the two major parties in Britain over Brexit has been widely treated as a temporary phenomenon, when the reality is that this collapse in party discipline is potentially significant and long-lasting. It may even usher in a new era of the party system. Jeremy Corbyn’s fall from grace and the fractious leadership race currently preoccupying the Labour Party demonstrate this ongoing problem quite clearly. And, despite Boris Johnson’s early success in mending fences in his Conservative Party, this newfound semblance of party unity will be severely tested as he moves on to the next Brexit challenge of securing an alternative trade deal.
result of the British debacle has surely been that the major political parties
in other EU member states have seen this unanticipated consequence, (along with
so many other negative developments related to Brexit), and are rethinking their
enthusiasm for departure.
 Livingston. Federalism and Constitutional Change. New York: Praeger, 1974.
 B. Jeffrey. Divided Loyalties: The Liberal Party of Canada 1984-2008. Toronto: University of Toronto Press, 2010.
 B. Jeffrey. Road to Redemption: The Liberal Party of Canada 2006-2019. Forthcoming University of Toronto Press.
 B. Jeffrey. “The Impact of Federalism on Intraparty Cohesion in Governing Parties: The Liberal Party of Canada and the British Labour Party”. Paper presented to the CPSA Annual Conference. Saskatoon. June 1, 2007.