The Liberal Democrats will reveal their new leader on Monday.

Jo Swinson is expected to defeat Ed Davey but the contest is unpredictable.

The winner will take over a resurgent anti-Brexit party that could be the kingmakers at the next election.

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The Liberal Democrats will on Monday afternoon announce the party's new leader — and potentially the all-important kingmaker at the United Kingdom's next general election.

The anti-Brexit party has experienced a surge of support, coming second in the European elections, ahead of both the governing Conservative party and opposition Labour party, gaining 700 seats in the local elections, and even topping some recent opinion polls.

Outgoing leader Sir Vince Cable will be replaced by one of the party's two most senior and experienced Members of Parliament — Jo Swinson and Ed Davey.

Here's everything you need to know.

Why does it matter?

With some polls putting the Lib Dems level — or in same cases ahead of — the Conservatives and/or Labour, the party is on course to significantly increase its number of House of Commons seats at the next election.

Outgoing leader Cable told Business Insider last month that the party was aiming to win another 60 seats. The party currently has 12 MPs after the high-profile defection of former Labour and Change UK MP, Chuka Umunna.

All polling indicates that the next election will produce another hung parliament, and in that scenario, the Lib Dems could well be in a position to form a government with either the Conservatives or the Labour Party.

Both Swinson and Davey have ruled out forming a Coalition with both the Tories or Labour in their current state. Any potential arrangement would be more informal, like a confidence-and-supply agreement, or on a vote-by-vote basis.

This means that the Lib Dems are not just choosing their new leader, but someone who could potentially hold the key to forming the UK's next government. Both have said they could be the next prime minister.

Crucially any deal with the other parties is likely only to come on the basis of them agreeing to hold a second Brexit referendum.

Who are the candidates?

Swinson, the MP East Dunbartonshire in Scotland, was first elected to the House of Commons in 2005 and was Westminster's youngest MP — the "Baby of the House" — until 2009.

When the Lib Dems went into Coalition with ex-prime minister David Cameron's Conservatives, she served as Parliamentary Under-Secretary of State in the business department, before later become a junior equalities minister.

She has played a major role in the anti-Brexit movement and is a leading supporter of the People's Vote campaign for a new referendum on whether the UK should leave the European Union.

Davey, the MP for Kingston and Surbiton in south London, also has experience of serving in government.

Like Swinson, Davey was a Parliamentary Under-Secretary of State in the business department during the Coalition years, before ex-prime minister Cameron made him Secretary of State for Energy and Climate Change in 2012.

He is also among the MPs who are part of the anti-Brexit movement.

So where do their campaigns differ?

Unlike the fractious Conservative leadership contest, the Lib Dem leadership race has been a clean, more peaceful contest, between candidates who agree on virtually everything.

Where the candidates differ is their personalities and leadership styles.

Supporters of Davey's leadership bid believe his biggest strength is his grasp of policy and argue that his plan for the country is more detailed and thought-through than what Swinson has put forward during the campaign.

At the forefront of his pitch is a plan to significantly increase efforts to fight climate change, in order to bring about "carbon-free capitalism." He has called for the end of internal flights in the UK, for example, and last month told Business Insider that he wanted the Lib Dems to overtake the Greens as the leading pro-environment party.

Swinson's backers, on the other hand, say her close ties with other anti-Brexit parties, combined with her impressive media performances, put her in a stronger position than Davey to win new voters and continue to grow the party.

Swinson has emphasised the importance of leading a new liberal movement, not just a single party.

Last month she told Business Insider that she was not a "tribal politician" and was best-placed to win "new voters across generations" and across "traditional party lines," as well as more MPs from other parties.

"I was down at the Donald Trump protest last week and several people came up to me and said they had joined the Lib Dems as a result of seeing me on Question Time," she told Business Insider. "It is working."

Who is going to win?

Swinson began the contest as the favourite and is widely expected to win on Monday.

However, unlike previous Lib Dem leadership contests, there is a greater degree of unpredictability this time around.

This is because the party has gained thousands of members since the last leadership contest, whose views on the leadership are pretty much unbeknown to party officials and the figures running the leadership campaigns.

One report on Sunday suggested that a sample of ballot papers in the race found the two candidates running at 50/50.

Nevertheless, multiple Lib Dem sources were this week predicting that Swinson would win — but by a margin much smaller than being suggested by some media reports and the betting odds offered by bookmakers.

A figure involved in one of the campaigns joked that the result could be 52-48 in favour of Swinson.

The winner will be announced at 16:00 (BST) on Monday.