The Liberal Democrats are to target pro-EU Labour supporters and “soft Tories” who backed remain, in a final canvassing blitz ahead of Thursday’s Richmond byelection, amid growing confidence in their camp that they are within striking distance of winning the seat.

Party documents obtained by the Observer, laying out their strategy for the final days, suggest undecided Labour voters will be key, and failure to persuade enough of them to vote tactically may prevent the Lib Dems pulling off one of the biggest byelection upsets of recent years.

Internal polling by the Lib Dems suggests they have closed the gap on former Tory MP Zac Goldsmith, a hardline Brexiter who quit to stand as an independent over the government’s decision to expand Heathrow airport, from about 20% at the start of the campaign, to under 4% with five days to go.

The Lib Dem strategy papers say undecided Labour voters have to switch if Goldsmith is to be ousted. “Canvass returns show that about a third of the remaining Labour support is open to us – winning this will be key,” the documents say. “There are still too many Lib Dem/Labour waverers ... and these Labour voters could hand victory to Zac Goldsmith if we cannot persuade them to vote tactically.”

Support for Labour, which decided to run a candidate, Christian Wolmar, despite pressure from senior figures in the party to give the Lib Dems a free run, has fallen from the mid teens at the start of campaign to just 9% now according to the Lib Dems.

These people and pro-EU “soft Tories” are concerned about Brexit and are therefore prime targets. “Unprompted, they raise Brexit as the deciding issue for them and they are positively engaging with our campaign on this issue,” the documents state. “How they break and whether or not the Labour vote drops another 2-3% will decide the result.”

The contest has been increasingly dominated by issues around Brexit. While Goldsmith has a strong local following, having fought a long if ultimately unsuccessful campaign to stop Heathrow expansion, he is at odds with the majority of his constituents over the EU. Seventy percent of people in the constituency voted for remain.

The Lib Dems’ own internal polling now puts Goldsmith on 46.7%, the Lib Dems on 43.3% and Labour on 9.5%.

Even if the Lib Dem candidate, Sarah Olney, was to come a good second to Goldsmith, it would give a significant boost to the party, which suffered a devastating general election result last year after five years in coalition with the Tories. Their total number of MPs fell from 56 to just eight.

Tim Farron, the leader of the Lib Dems, has highlighted the risk that Brexit and the loss of EU workers would represent to the local health service, claiming that about 250 doctors and over 1,000 nurses in the area were from other countries inside the EU, and might have to return home. At Kingston hospital over a third of nurses and almost one in five doctors are EU migrants, he said.

“A hard Brexit would stretch the NHS to breaking point,” Farron said, “ blowing a hole in the public finances and risking an exodus of EU nationals on which our health service relies. Hospitals around Richmond, where hundreds of doctors and over a thousand nurses come from Europe, would be particularly badly hit.”

The Greens’ sole MP, Caroline Lucas, joined the Liberal Democrats on the campaign trail in Richmond on Saturday, where she revealed she would vote against the triggering of article 50, saying she cannot “throw the country into the potential nightmare of a hard Brexit within two years”.

She said: “A regressive alliance of the Tories and Ukip are working together to reinstall a pro-Brexit MP. We have stepped up at this unique moment and are backing Sarah Olney as the best hope of denting the government’s plans for a painful Brexit from the European Union.”



Lucas said the government’s refusal to offer a second referendum on the terms of any deal had left her with no option but to oppose article 50 in the Commons.



Lucas, whose Brighton Pavilion constituency backed remain by a margin of 68% to 32%, went on to accuse the government of turning “a marginal vote in favour of leaving the EU into a phantom majority that wants us out of the single market – and all of the benefits it entails”.

She said: “I’m proud to have fought as hard as I could to stop Britain from leaving, but I accept that we lost. But without any clarity as to what kind of Brexit the government intends and without any plans to call an election or offer a referendum on the terms of any deal, I cannot – as a democrat and someone who believes in social and environmental justice – vote to throw the country into the potential nightmare of a hard Brexit within two years.

“The effects of a chaotic, hard Brexit would be devastating: jobs lost across the UK, our environmental and social protections torn up, and EU citizens living here could be left in limbo.”

Lucas, a vocal proponent of a progressive alliance between the Greens, Labour and the Liberal Democrats, went on to accuse Labour’s leadership of capitulating to May by refusing to vote against article 50.

Last week, John McDonnell, the shadow chancellor, said the party would not “seek to block or delay” the triggering of the EU’s exit mechanism. Keir Starmer, the shadow Brexit secretary, has also said Labour would not “frustrate the process by simply voting down article 50”.

Labour’s position effectively gives the government carte blanche to push through a hard Brexit, according to Lucas. She said: “It’s extraordinary that ministers expect MPs to simply fall in line without knowing what we’re voting for, and it is truly puzzling that the Labour party has already capitulated by pledging to vote in favour of triggering article 50 – thus handing the government far more power to push through their agenda.”

Of Labour’s 231 MPs, only five – including former leadership candidate Owen Smith and Tottenham MP, David Lammy – have said they would defy the party whip and vote against article 50 alongside the Liberal Democrats and a handful of Tory rebels such as Ken Clarke. The SNP’s 54 MPs are also highly likely to join them, in a move Lucas said was “in the country’s interests”.

She said: “I still believe that Britain is better off as part of the European Union, and I’ll be campaigning in the next election for our continued membership of the biggest peace project in history.

“As a constituency MP and co-leader of a national party, I believe that I have a duty both to represent my constituents and to act in the country’s interest – and I firmly believe that voting to trigger article 50, with things as they currently stand, runs counter to both of those roles.”