Zac Goldsmith’s decision to resign as a Tory MP and contest his Richmond Park seat in a by-election as an independent, in protest at the government’s decision to build a third Heathrow runway, has raised the thorny issue of electoral pacts. With the Conservatives and UKIP choosing not to contest the seat, Labour came under pressure to promote one candidate of the anti-Brexit left.

MPs Clive Lewis, Lisa Nandy and Jonathan Reynolds all urged Labour to consider standing aside in favour of a Lib Dem candidate in order not to split the anti-Brexit vote. This way, they argued, the by-election could be turned into a referendum, not on Heathrow expansion, but on the xenophobic politics that Goldsmith symbolises.

This called was echoed by an article in the New Statesman by Neal Lawson, the leader of the centre-left pressure group Compass. Standing aside would be a “history making decision” for Labour and “a famous turning point in the politics of our nation,” he claimed.

Labour is, however, contesting this by-election, which is by far the wisest course of action. After the unnecessary internal battles of the summer leadership election, which has damaged our Party’s standing in the polls, the last thing we need now is a divisive strategic leap into the dark on electoral alliances.

But the pressure will continue to mount. The Lib Dems, virtually wiped out in the 2015 general election, with fewer than 10% of the vote and only eight MPs, appear to be back in business. Having dumped Nick Clegg in favour of a less pro-Tory leader, they grabbed 30% of the vote in the recent Witney by-election, knocking Labour into third place. With a Conservative overall majority of just a dozen MPs and Brexit the new fault line in British politics, Labour will be under ongoing pressure to ensure there is only one anti-Tory candidate in future by-elections and even in marginal Conservative seats at the next general election.

What’s alarming about the contributions quoted above is the assumption that an electoral deal to unify the anti-Tory vote should automatically take the form of Labour not standing. Arguably, in many of the Tory heartlands, the Lib Dems will always look like the party best placed to unseat a sitting Conservative MP. To surrender to this pressure would mean no Corbynist message in these areas, the disenfranchising of thousands of Labour voters and resuscitation of a party, which, while sometimes opposing Tory policy, is no friend of Labour, as the last forty years of political history prove.

A far better approach than surrendering “unwinnable” seats to the Lib Dems is to look at ways of neutralising their impact. One suggestion that surfaced over the Richmond Park by-election was the idea of an open primary. Writing in The Guardian, Hugh Dixon argued, “The real way to revitalise our democracy would be for the Liberal Democrats, Labour and the Greens to agree to back a common candidate – chosen through a primary in which all pro-Europeans living in the constituency can vote.”

This is an altogether more interesting proposition. Open primaries are widely used in the US to select parties’ candidates. But that is not quite what is being suggested here. Suppose, for example, a deal were to be struck between Labour, the Lib Dems and the Greens, in a number of tricky Tory seats, whereby each party could select a candidate, who would then be put into an open primary where any voter in the constituency could take part, to select the most popular of the three to go head to head against the sitting Tory.

Pessimists would argue that media pressure and natural caution might lead voters in these constituencies automatically to plump for the most moderate anti-Tory candidate, invariably the Lib Dem. I’m not so sure. Labour’s membership has pretty much trebled in the last eighteen months. In Momentum we are building a formidable organisation that can deliver results on the ground. It was their mobilisations in last year’s Oldham by-election that helped turn around wobbly opinion polls and hold the seat for Labour with a comfortable majority. Even in Tory-held seats, in an open contest to pick a united anti-Tory candidate, it’s quite possible that Corbyn rallies and grassroots organisation by this huge new network of support we are constructing could see off the Lib Dems and give Labour a clear run at the polls.

Of course there are organisational difficulties involved. Getting the other parties to agree to – and abide by the result of – such an arrangement may prove intractable. But at a time when the left is often accused of talking too much to itself, this could be an ideal opportunity to take our message into hitherto uncharted territory, with a real prospect of winning. It’s an idea to be considered, at any rate.

Longer term, the most effective way to keep the Lib Dems at bay is to occupy their political territory. Even Tony Blair understood this when he ransacked their proposals for constitutional reform and made them his own. But the same Blair, whose authoritarian crackdown on long-held freedoms and illegal invasion of Iraq gave the Lib Dems the space to pose both as defenders of civil liberties and as an anti-war party.

If we are to win the next election, Labour has to be the force that unifies all the anti-Tory feeling being generated across the country. We have to be the real greens and the real liberals too – staunch defenders of civil liberties, the Human Rights Act, personal freedom from state intrusion, for wide-ranging constitutional reform, Europeanism and internationalism. In the process we can build the Party as a real social movement across the country and in areas where Labour’s traditional messages have not always resonated.