By Rebecca Pinnington

Ribble Valley in Lancashire may have been one of the safest Conservative seats in the country, but in 1991, the voters were irate. Public outrage over Thatcher's poll tax, which John Major had been slow to abolish despite widespread protest and promises of change, led the Liberal Democrats to a storming victory, overturning Conservative Nigel Evans' 20,000 vote majority to win the seat. Two weeks later, the poll tax was scrapped.

The link between the Lib Dems' victory in Ribble Valley and the abolition of the poll tax is not a straight line – after all, John Major had already promised change – but it is widely viewed as one of the key catalysts for the abrupt about-face. Now, the Lib Dems are hoping to trigger another policy change with a victory in Richmond Park and North Kingston this Thursday. Again, they are running on the basis of opposition to a major government policy, and will need to demolish a 20,000 vote majority – this time, that of former Conservative MP and Brexiteer Zac Goldsmith.

Goldsmith resigned both his seat and the Tory whip in October in protest against the government's planned expansion of Heathrow airport, which has a flight path over the constituency. He claimed the by-election would be a referendum on the plans, but it hasn't quite worked out that way. Had the Conservatives fielded a candidate against him, Goldsmith could have capitalised on anti-Heathrow sentiment to deal a blow to the government; but all of his viable rival candidates agrees with him on Heathrow.

The Lib Dems, on the other hand, are determined this will be a referendum on the way the government is handling Brexit – and they could get their way. Some 70% of Richmond Park voters wanted to remain in the EU in June, and the Lib Dem candidate, Sarah Olney, claims it continues to be a huge concern. "Voters are responding more to Brexit, I won't beat about the bush," she says. "Because all of the main candidates standing are anti-Heathrow expansion, it's not affecting how people are going to vote. Brexit is affecting how people are going to vote."

If voters reject Brexit and the direction in which the government is taking it, as the Lib Dems are encouraging them to do, Olney's supporters hope it will send a clear message to the government that it cannot pursue an isolationist hard Brexit, and will have to consult on the terms of leaving the EU. "A win here could have all sorts of impacts," Olney suggests. "I think it would show that there is grassroots support for a continuing remain campaign and for maintaining links with Europe. Once we've got that momentum here we can take that forward and we can start to take the campaign round the rest of the country." Out canvassing for Olney, Ruth Dromey, the Lib Dem leader in nearby Sutton, agrees. "If we manage to win and pull this off and Sarah becomes the next MP for Richmond Park, that will be a very strong statement to the government about Brexit."

A Lib Dem win in Richmond Park and North Kingston would be a statement, certainly, but whether it would bring about the kind of change these campaigners would like to see is another matter. There's a chance Theresa May's government could dismiss the result as the consequence of Goldsmith being 'damaged goods' after his widely-criticised mayoral campaign. Similarly, they could dismiss it as a poor indication of the mood of the rest of the country, given that the overwhelmingly affluent and upper middle class are hardly representative of the country as a whole. Former energy and climate change secretary Ed Davey reflects that, while the government would have to consider the impact of a Lib Dem victory, or even a large swing towards them, "It doesn't necessarily lead to something changing because, on the other side of it, there's been a referendum result.

"What will worry them," he says, "is we're finding that a lot of Conservatives who've never voted Liberal Democrat are going to vote Liberal Democrat this time. If the Tories then look at a lot of their seats where there was a high remain vote – and there were a lot of Tory seats with a high remain vote – they are going to get worried."

At a public meeting in the constituency, party leader Tim Farron is more confident. "By-elections do not have a habit of changing governments, but they do have a habit of changing the direction of governments," he states. "Because whatever we think about our prime minister and about this government, they are sensitive to public opinion and they would be foolish to do otherwise." While Theresa May may be listening to right-wing voices within her own party at the moment, she will listen to centre-left voices wanting a change in direction if she loses this safe seat, he says.

"So no hyperbole from me when I say, this is the most important parliamentary by-election in living memory. The people of Richmond Park and North Kingston are the most powerful people in the United Kingdom at the moment, and there are five days to go to change the direction of our country. […] On Thursday this is about electing a good local MP but it's also about the kind of country we want to live in.”

Whichever candidate wins, Olney or Goldsmith, Richmond Park is set to be a close by-election, and its result is hotly anticipated by both sides. If Goldsmith is re-elected, the status quo will remain intact, and Remainers will have to find other means of forcing the government’s hand.

If, on the other hand, the Lib Dems succeed in dismantling Goldsmith's majority in a perceived safe seat, the Lib Dems could undermine the government on Brexit and force Theresa May into concessions. Like Ribble Valley, this would be a major turning point in British politics.

Rebecca Pinnington is a freelance journalist. You can follow her on Twitter @beckypinners

The opinions in's Comment and Analysis section are those of the author and are no reflection of the views of the website or its owners.