After leading the British Labour Party into its worst electoral defeat since 1935, Jeremy Corbyn is stepping down as leader.

The contest to succeed him will take three months and pit defenders of Corbyn’s legacy against centrists who believe the party must change.

Here is everything you need to know.

Schedule

The election consists of three stages:

1. January 7-13: Members of the House of Commons and the European Parliament nominate fellow lawmakers. Candidates must have the support of 10 percent of lawmakers to qualify.
2. January 15-February 14: Constituency parties and affiliates — trade unions and socialist societies like the Fabian Society, Labour Business and LGBT Labour — make their endorsements. Candidates must have the support of at least 5 percent of Labour’s constituency parties or three affiliates, of which two must be trade unions, representing at least 5 percent of the affiliated membership. These requirements were introduced under Corbyn under the guise of empowering the grassroots but really to make a parliamentary leadership coup more difficult.
3. February 21-April 2: Party members, affiliate members and registered supporters vote by mail.

The winner is due to be announced on April 4, which would give Labour a new leader before the local elections in May.

Who can vote?

• Members of the Labour Party
• Members of affiliated trade unions
• Members of affiliated socialist societies
• Registered supporters

Members of affiliated trade unions and socialist societies need to opt in to vote.

Party membership rose dramatically under Corbyn’s leadership from under 300,000 in 2015 to almost half a million.

In 2015, non-members were allowed to register as supporters and vote for a £3 fee. Around 100,000 did.

In 2016, when Owen Smith challenged Corbyn, registration was open for only two days and the cost to vote raised to £25. Still some 180,000 joined, many of them Corbyn supporters.

The same rules apply this year. Registration will be open between January 14 and 16 and cost £25.

People will have until January 20 to join the Labour Party or one of its affiliates as a full member.

This splits the difference between Corbyn loyalists and critics. The former want to prevent an influx of new members — much like they themselves joined the Labour Party en masse in 2015 and 2016 to vote for Corbyn. The latter want to give disaffected Labour Party supporters ample time to sign up.

How does the election work?

The party uses preferential voting. Voters rank the candidates in order of preference. If more than half the voters rank the same candidate as their favorite, he or she wins. If not, the candidate who places last is eliminated and their votes are redistributed to the voters’ next preference. The process is repeated until a candidate has more than 50 percent support.

Who are the candidates?

Five candidates have declared so far.

• Keir Starmer is the favorite. Relatively centrist and passionately pro-EU, he is nevertheless trying to split the difference between pro- and anti-Corbyn members by arguing Labour must not “oversteer” away from the outgoing leader’s radicalism.
• Jess Phillips has been one of the most outspoken critics of Corbyn’s leadership, in particular his failure to tackle antisemitism and bullying. She is also one of the least ideological candidates. Liberal commentator Matthew D’Ancona believes that makes her the strongest contender: “Her appeal lies in a refreshing candor, humanity and ability to talk as a normal person rather than in the stilted phrases of most professional politicians.”
• Lisa Nandy has been tipped as a possible future leader for years. She resigned from the shadow cabinet after the EU referendum, accusing Corbyn of campaigning against Brexit halfheartedly. Her focus has been on winning back Labour voters in small towns.
• Emily Thornberry broke with Corbyn on Brexit but has otherwise been one of his most loyal supporters.
• Clive Lewis, an Afghan War veteran, backed Corbyn from the start but was removed as shadow defense secretary when Corbyn refused to press his anti-nuclear policy.

Four others have expressed an interest in running.

• Rebecca Long-Bailey, a protégé of shadow chancellor John McDonnell, would the continuity candidate. She could count on the support of Corbyn’s allies, including the large trade union Unite.
• Ian Lavery, the party chair and another Corbyn ally. If Long-Bailey doesn’t run, Lavery, a former union man himself, could win Unite’s endorsement.
• Yvette Cooper was one of the candidates who lost to Corbyn in 2015. An ally of former prime minister Gordon Brown, she is on the right of the party.
• Dan Jarvis, a former parachuter, is also considered a centrist. He campaigned to keep Britain in the EU and, with 66 other parliamentarians, defied Corbyn to vote for airstrikes against the self-declared Islamic State in Syria.

Who is ahead?

Only one poll of party members has been conducted so far, by YouGov. It puts Starmer in the lead with 31 percent support, followed by Long-Bailey at 20, Phillips at 11, Cooper and Lewis at 7, Thornberry at 6 and Nandy at 5. Starmer is also the second or third choice of many members, which would allow him to defeat Long-Bailey with 61 to 39 percent support in a final voting round.

Starmer also leads in terms of endorsements from lawmakers with seven declared supporters so far. Phillips is in second place with six, Nandy has four and Lewis one.