Five MPs formally entered the race on Monday to succeed Jeremy Corbyn as leader of Britain's main opposition Labour party, and rebuild their movement after last month's disastrous election.
Keir Starmer, a former director of public prosecutions who as Brexit spokesman helped shift the party to campaign against leaving the EU, is currently favorite among lawmakers.
But his closest contender, avowed socialist Rebecca Long-Bailey, is tipped to benefit from her closeness to Corbyn and support among left-wing activists when the race opens up to party members.
Who wins will be crucial in determining whether Labour can recover from its worst election result since 1935, and take the fight to Prime Minister Boris Johnson's Conservatives.
The next election is not until 2024 but a strong opposition could challenge the Tories as they reshape the British economy and beyond after Brexit.
Corbyn announced his resignation after the Dec 12 election, which unleashed a fierce internal debate about Labour's direction that has spilled over into the leadership campaign.
Starmer is viewed as a more centrist figure who will move the party closer to where it was under Tony Blair, who was prime minister for a decade until 2007. But while his left-wing critics deride him for this - Blair remains deeply unpopular because of the Iraq war - Starmer has emphasized his progressive credentials.
He says he would keep elements of Corbyn's policy program, notably the Green New Deal to transform the economy to face the challenges of climate change, and address the failed free market model.
He highlights his years as a lawyer defending workers and environmental protesters, and later as chief state prosecutor in England and Wales. In that role, he took on MPs over expenses abuse and newspapers over phone hacking.
Starmer has the backing of Britain's largest trade union, Unison, and is the most popular candidate among MPs and members of the European Parliament. He received 88 nominations before Monday's deadline.
By contrast, there is no disputing Long-Bailey's commitment to continue Corbyn's legacy. The party's business spokeswoman and a close ally of the current leader, she recently gave her boss 10 out of 10 when asked to rate his leadership.
She backs his radical agenda, with its plans to nationalize key industries and invest huge sums in public services, and also wants to abolish the House of Lords.
A trained lawyer, she argues that Labour needs a proud socialist at the helm, and has promised to fight the establishment tooth and nail.
Long-Bailey has been endorsed by left-wing activist group Momentum, which grew out of grassroots support for Corbyn in 2015, and has secured the nominations of 33 MPs and MEPs.
Three other women are also hoping to be Labour's first female leader in its nearly 120-year history. Lisa Nandy, who has been outspoken about the need to respect the Brexit vote and help forgotten communities, secured 31 nominations. Foreign affairs spokeswoman Emily Thornberry, a leading Brexit critic, and outspoken backbencher Jess Phillips both secured 23.
Candidates must now get the support of 33 local Labour associations or three party affiliates, including at least two trade unions. They will then go into a final postal ballot of party members and supporters who paid to take part, which runs from Feb 21 to Apr 2. The winner will be announced on Apr 4.