If you’re happy with the prospect of a Conservative government led by Boris Johnson, read no further. This article-cum-rant is not for you. But if, like me, you’re terrified of the hard Brexit and Trump assault on our NHS that would inevitably follow, please read on. I’m going to ask some of you to do something you may never have contemplated before.

I used to deliver leaflets for my Liberal councillor father but voted Labour. Now, in a sense I’m doing the opposite. I’m a traditional Labour voter living in Lewes, East Sussex, where even the most optimistic Corbynista recognises that the most effective way to stop Johnson wreaking havoc on our country is to vote Liberal Democrat. I have met their candidate, Oli Henman, a young man with a beard, whom I liked. I told him I would vote for him. I don’t think I have ever voted Lib Dem before, but I understand how vital it is in this election to “box clever” and vote tactically. I’m hoping in my own small way to set an example to other Labour supporters, however uncomfortable they may find it to break their traditional loyalty.

But no armchair preaching from me. On Sunday I went to Hastings to campaign with Labour and there I urged Liberal Democrats, Greens (and one-nation, moderate Conservatives) to vote Labour. The Labour candidate, Peter Chowney, is a very likable middle-aged man (also with a beard) who leads the council. Labour are seeking to overturn the Conservatives’ 2017 majority of 436 in what used to be Amber Rudd’s Tory seat, and the new Tory candidate is being investigated for alleged antisemitism. She is also known for implying that below-minimum-wage salaries for learning-disabled workers are acceptable because “they don’t understand money”. The nasty party is well and truly back. Happily, a busload of Labour activists from Lewes had also arrived in Hastings to try to help Labour win.

On Monday evening I headed to Chipping Barnet, north London, where the Labour candidate, Emma Whysall, is close to winning that seat from another Brexit hardliner Tory, Theresa Villiers. Her majority in 2015 was fewer than 400 votes, and I urged Lib Dems and Green to lend Emma their vote to get her over the line. Then I went on to St Albans, where another rightwing Brexiteer, Anne Main, won this firmly remain seat by just a few thousand votes in 2017 from Daisy Cooper, a Lib Dem and still the main challenger for the seat. Daisy’s views are as progressive as mine, though her personal life is probably less eventful. She is the only candidate who can unseat the Tory, and there is a sizable third-place Labour vote who should back her to ensure we have one more MP who will campaign vigorously against Brexit and for social justice.

Facebook Twitter Pinterest Daisy Cooper, the Liberal democrat candidate for St Albans, pictured with Jo Swinson, ‘is the only candidate who can unseat the Tory, and there is a sizeable third-place Labour vote who should back her’. Photograph: Vickie Flores/EPA

We have seen Nigel Farage and Johnson essentially making a pact (though pretending not to) while Labour and the Lib Dems continue to bite chunks out of each other, unable to cast aside past rivalries. But we – the voters – can triumph over tribalism if we live in a marginal seat and are prepared – this once – to lend our vote to the candidate best placed to defeat an incumbent Tory. If you’re not sure which is the best-placed candidate in your constituency, there are several online tactical voting sites.

Tactical voting across the UK: a region-by-region guide Read more

Some in the Labour party, the party I support, may not be happy about my advocating widespread tactical voting. I know that, and I understand that in some cases a tactical vote will mean ignoring some policy differences past and present. But, guys, come on! Imagine waking up on Friday morning and realising that, were it not for the well-intentioned idealism or the overcooked obstinacy of a few thousand voters whose preferred candidate never stood a chance, we could have avoided five years of misery under Johnson. Perhaps, when Labour throws its weight behind reform of the electoral system, and so abolishes any need for tactical voting, I may accept that the country’s interests lie in unbending loyalty to the party. In the meantime, fellow voters, let’s all use our heads rather than just our hearts. Please, if you live in one of those marginal seats, lend your vote to the candidate who could help stop Boris Johnson and avoid a hard Brexit.

• Steve Coogan is a comedian, actor, writer and producer