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Lib Dem chiefs believe they can win an unprecedented Commons majority with a secret “Project 320” plan.

Party leader Jo Swinson has written to members outlining how she believes it can become the biggest group in Parliament and propel her to No10.

Headed “confidential briefing … building a liberal majority”, the leaflet begging activists for cash reveals a plot to scoop 320 Westminster seats at the looming ballot and give the party an outright majority.

“We will be fighting the General Election win... From our strengthened position in the polls, we can challenge to win a General Election,” says the document.

“We go into a 2019 General Election in our strongest ever position – largest ever membership, best ever EU Election results, best ever council election results and having won the Brecon & Radnorshire by-election cutting Boris Johnson ’s majority to just one.”

A separate letter adds: “Winning and seizing the opportunity we face will take a campaign effort the likes of which we have never seen before - and by joining Team 320, you’ll be a key part of making that liberal majority a reality.”

Pleading with supporters to inject funds, the letter says a £100-a-month gift would help the party “produce canvassing packs to knock on 4,000 doors in a constituency every month”.

Far from being in its “strongest ever position”, the party trails the Conservatives and Labour in national polls.

(Image: REUTERS)

It has just 18 MPs – six of whom are defectors originally elected for Labour or the Tories.

That compares with the 2005 election where, led by Charles Kennedy, the Lib Dems won 62 seats, and the 2010 election campaign where, with Nick Clegg at the helm, they briefly surged to 34% in polls - ahead of Labour and the Conservatives.

Ms Swinson, 39, has been bullish about her unlikely chances of becoming Prime Minister.

Last month, as she prepared for her party conference debut as leader, she insisted to the Mirror she was ready for Downing Street.

Asked he she could really become PM, she said: “Absolutely – and what’s more, I think I would be a better Prime Minister than either Boris Johnson or Jeremy Corbyn .

(Image: Philip Coburn)

“I say that with complete confidence because I don’t think either of them is up to the job.”

But the party trails its two big rivals in donations and membership.

Latest figures show that in the three months to June, donors pumped £5.4million into the Tory war chest, £5million into Labour coffers and just £1.9million to the Lib Dems.

The Lib Dems have 120,000 members, compared with 180,000 in the Conservatives and 485,000 in Labour.

The party also faced criticism of its new policy to scrap Brexit without a second referendum.

Previously it backed another EU vote.

But its conference in Bournemouth agreed a plan to simply axe withdrawal by revoking Article 50 - the formal mechanism for quitting the bloc - without asking voters, prompting claims the party was undemocratic.

A senior Lib Dem source said: “The Liberal Democrats beat the Conservatives and Labour in a national election for the first time in a century in May.

“We have record membership and a leader whose popularity regularly outstrips the popularity of Jeremy Corbyn.

“Over the past few years people have been crying out for a new progressive party.

“With a new leader, new MPs and tens of thousands of new members, we are that party.”