The process for selecting the next Labour Party leader—and potentially the next prime minister—illustrates a unique and complex electoral structure, nearly half a century after party members first gained direct voting rights in leadership contests. Should a challenge to Sir Keir Starmer arise in the near future, it would mark a significant milestone, as Labour members have rarely been given the opportunity to decide who leads both the party and the country.

The Labour Party initially expanded its leadership franchise beyond the parliamentary caucus in the 1980s, but extended periods in opposition and an uncontested leadership election in 2007 limited members' direct influence over the prime ministerial selection. In contrast, the Conservative Party’s 2019 leadership election brought a sharper focus on the role of party members, whose demographics—predominantly older, middle class, male, and white—led to criticisms that an unrepresentative base dictated the country’s leadership. Labour’s membership mirrors this demographic profile closely, prompting some observers to highlight a mirrored dynamic of an entrenched and potentially unaccountable group influencing leadership selection on the left.

However, the Labour Party’s voter base for leadership elections is broader than just party members. Affiliated supporters also participate, primarily trade union members who contribute a political levy to allow their unions to engage politically. This group constitutes a substantial portion of the electorate. For instance, in the Labour deputy leadership election last year, the electorate numbered over 970,000—a figure significantly larger than the estimated 250,000 party members—and roughly three-quarters of voters were non-party members affiliated through unions.

Critics argue that expanding the franchise beyond MPs and party members risks producing leaders who lack the confidence of the parliamentary party, as seen historically with Jeremy Corbyn and Liz Truss on the Conservative side. Labour’s electoral system, however, can create a scenario where the selected prime ministerial candidate lacks strong support from either MPs or members. Although affiliated supporters traditionally have had low turnout—often under 17 percent, indicative of low union participation—their votes have historically aligned with party members when they do engage.

Notably, there have been exceptions. In the 2010 leadership contest, the affiliated voters tipped the balance in favor of Ed Miliband, despite the parliamentary party and rank-and-file members preferring his brother, David. This illustrates the potentially decisive influence of union-affiliated voters, who have until now never directly chosen a prime minister.

Looking ahead, trade unions may intensify efforts to mobilize these affiliated voters, who remain difficult to understand and engage due to data protection concerns and restrictions prohibiting direct candidate communication. As a result, polling rarely captures this group accurately, leaving their preferences largely unknown.

The trajectory of Labour’s next leader could still be determined without a broad contest if Andy Burnham wins the upcoming Makerfield by-election—a highly symbolic political battle that, if successful, could prompt Starmer’s departure and yield uncontested leadership acclaim. However, if the leadership race proceeds, it will represent the largest party franchise ever assembled to directly elect a British prime minister, highlighting the evolving and distinctive nature of internal Labour Party democracy.