In the post-Brexit era of the 21st century, the political landscape of the UK has undergone profound restructuring, with the traditional two-party system loosening. The voter bases of both major parties have shrunk, with their combined vote share dropping from 96.8% in 1951 to 57.4% in 2024, a century-low. The 2024 general election and 2025 local elections revealed deep-seated degradation in the two-party system's operational mechanisms, marked by distorted constituency dynamics, fractured political representation, and the rise of new forces. The first-past-the-post system produced skewed results: Labour won nearly two-thirds of parliamentary seats with just over one-third of the vote, while smaller parties secured far fewer seats than their vote shares warranted. Political realignment transcended traditional geographic boundaries, with the Reform Party replacing the Conservatives as Labour's main rival in previously "safe" Labour constituencies. Voter loyalty to parties eroded rapidly, with turnout hitting a historic low of 59%. Conservative voters defected to the Reform Party, Labour voters split their support with the Greens, and over 30,000 voters abstained. The axis of political division shifted from class conflict to debates over values and cultural issues, a transformation accelerated by Brexit and solidified in the post-Brexit era. In the 2025 local elections, the Reform Party won nearly 30% of the vote, securing 677 seats, outperforming both the Conservatives and Labour, controlling 10 local councils, and winning two mayoral races—marking the first substantive break from the traditional two-party dominance and signaling a profound shift in UK local politics. In the Runcorn-Helsby parliamentary by-election, the Reform Party defeated Labour to claim a key seat, dealing a heavy blow to Labour's traditional stronghold. The 2024 general election saw the Conservatives' vote share hit its lowest since 1832, while Labour secured 63% of seats with just 34% of the vote. The Reform Party's surge in the 2025 local elections disrupted the power dynamics of the major parties, underscoring the stagnation facing the UK's traditional two-party alternation model.

I. Basic Concepts and Research Questions

The discussion centers on the "Westminster system" in UK politics and its impact on the two-party system, including three theoretical pillars: Duverger's Law, the Alford Index, and the intergenerational transmission of party identification. These theories explain the stability of 20th-century British politics. However, the 21st century has witnessed dramatic changes in the UK's party landscape, such as frequent hung parliaments, the rise of regional parties, and the fragmentation of voter bases. The 2010 general election coalition government broke the tradition of two-party alternation, while the 2017 and 2019 elections further weakened mainstream parties. The rise of new forces like the Scottish National Party, declining voter loyalty, and issue-driven voting behavior have sparked debates about the decline of the UK's two-party system. This paper analyzes the 2024 general election and 2025 local elections to explore the challenges facing the two-party system and the emerging trends of multipolarity and coalition politics in the UK's political system.

II. The 2024 UK General Election: Party Dynamics and Voter Behavior

The 2024 UK general election marked a significant shift in the political landscape, with Labour winning a parliamentary majority of 411 seats but securing only 33.7% of the vote, while the Conservatives suffered their worst defeat in history, dropping to 23.7% of the vote and just 121 seats. The disproportionality of the electoral system resulted in a Gallagher Index of 23.6, a record high. The Reform Party and Liberal Democrats performed strongly, while the Greens and Scottish National Party faltered. The outcome stemmed from crises during the Conservatives' final term, including the "Partygate" scandal, the financial market turmoil triggered by the "mini-budget," and controversies over the "Rwanda Plan." Under Keir Starmer's leadership, Labour adjusted its policies, abandoning contentious left-wing proposals to focus on economic growth and public services. Voter behavior showed high volatility, with Brexit declining in importance while culture-war issues emerged as new political dividing lines. The increase in the effective number of parties signaled the de facto collapse of the two-party system, and record-low turnout reflected public dissatisfaction with the political establishment.

III. The 2025 England Local Elections: Party Dynamics and Voter Behavior

In the 2025 England local elections, the right-wing populist Reform Party achieved a landslide victory as the biggest winner, followed by the Liberal Democrats, while the Conservatives and Labour suffered historic defeats. The election ended a century of two-party dominance in English local politics, reflecting a shock to the traditional party order. Public dissatisfaction with Labour's Starmer government surged due to broken campaign promises and controversial economic and social policies. After its general election disaster, the Conservative Party continued to hemorrhage support, with at least 80 former MPs, candidates, donors, and senior aides defecting to the Reform Party. Local council control shifted dramatically, with the Reform Party gaining control in multiple counties. The distorting effect of the first-past-the-post system reached new heights, as the Reform Party won 45.9% of seats with just 32.3% of the vote. The contrast between electoral fragmentation and the two-party facade in parliament highlighted the structural disconnect between the electoral system and pluralistic public opinion. The 2025 local elections made clear that the UK's representative democracy urgently needs electoral reform to adapt to these political changes.

IV. Analyzing the Causes of the UK Two-Party System's Stagnation

Since the 21st century, the UK's two-party system has faced five major challenges: the failure of the core feedback mechanism ("thermostat effect"), the distortion of voter accountability under first-past-the-post, structural biases in the electoral system, shifts in societal cleavages, and the rise of the right-wing populist Reform Party. Together, these factors form a comprehensive explanatory framework for the challenges to the UK's party-political landscape.

The failure of the "thermostat effect" has transformed the two-party system from a "pendulum model" to a multiparty "chaotic state." Policy polarization between the two major parties has weakened their appeal to centrist voters, fueling the rise of single-issue parties like the Liberal Democrats. Multiparty competition has reshaped the strategic logic of UK party politics, leaving the two major parties in a dilemma. Voter behavior has also qualitatively changed, with more people opting for "protest voting" to express single-issue stances.

Under first-past-the-post, the fragmentation of the party system is closely linked to the effectiveness of voter accountability. The core of voter accountability theory lies in the dual political functions embedded in a single vote. For non-centrist voters, this contradiction in dual political demands within a single voting tool inevitably leads to dilemmas. Voter attitudes toward their preferred parties have become more transactional, with left-wing parties like the Greens, Scottish National Party, and Plaid Cymru, as well as right-wing parties like the National Front, British National Party, UKIP, and Reform Party, attracting significant protest votes.

The catalytic effect of electoral bias mechanisms has shaken the foundations of the UK's two-party system. Electoral bias refers to systemic unfairness in first-past-the-post systems, where structural misalignment prevents equal vote shares from translating into equal seat shares. This bias manifests as vote distribution efficiency bias, turnout bias, third-party vote bias, and constituency size bias. These four mechanisms collectively create an electoral bias that transcends fair intent, perpetuating a disconnect between seat allocation and public support.

The weakening of class voting has destabilized traditional voting blocs. Brexit reshaped this landscape in an unprecedented way. Many traditionally Labour-leaning, pro-Brexit voters switched to the Conservatives over sovereignty and immigration issues, while many pro-Remain, traditionally Conservative middle-class voters defected to Labour over economic concerns. This realignment caused class voting patterns to erode rapidly. Age and education differences are closely tied to class voting but also exert independent influence. Younger, highly educated voters tend to support Labour, the Liberal Democrats, or the Greens, while older, less-educated voters favor the Conservatives and Reform Party.

V. Conclusion

UK politics is undergoing profound transformation, with the structural foundations of the two-party system weakening, evidenced by historic lows in combined vote share, shifting voting factors, and declining public trust. Labour's 2024 landslide was not a revival of the two-party system but a brief prelude to change. The new normal in UK politics may be a fragmented landscape of five major parties competing intensely, exhibiting some stability. The 2029 general election could produce another hung parliament. The two-party system, as a UK political paradigm, is stagnating, and future change is inevitable. Traditional political theories—such as Sartori's typology of party systems and the left-right model—are inadequate and require innovation. Electoral geography must also be rethought to account for emerging parties' regional penetration and constituency-type shifts. Understanding voter spatial distribution, decision-making, and emerging parties' base-building has become crucial. These theoretical innovations offer analytical perspectives for observing political fragmentation in other Western democracies. The changes in UK party politics reveal the crises facing Western representative democracy amid economic inequality, sociocultural conflict, and collapsing public trust. Outdated institutional designs may exacerbate polarization, representation crises, and government illegitimacy, breeding populist and extremist forces. Institutional design must evolve with societal and political participation changes—or risk becoming a source of instability.