Comedian Steve Coogan said people needed to "vote smart and box clever" to keep the Conservatives out of power.
The Alan Partridge star took to the streets in his local constituency of Lewes in East Sussex with Lib Dem candidate Oliver Henman on Sunday to try to unseat the Tory party.
Conservative Maria Caulfield won with a majority of 5,508 in 2017 and people narrowly voted to remain during the 2016 referendum.
Speaking to campaigners, a bearded Coogan said that although he was a "traditional Labour supporter" he would be voting Lib Dem on Thursday because it was the "smartest thing to do".
But Coogan said he will be visiting other constituencies where he would urge people to vote Labour in order to keep the Tories out if they were the main opponent.
He added: "It's such a crucial issue facing the nation.
"The most important thing across the country is to defeat the Tories by any means necessary. It's about being smart and boxing clever."
The comedian, who sported a "tactical voting vital" sticker on his jacket, joked he had grown a beard to be more like a liberal but said it was just for the campaign.
Coogan then went round the streets canvassing with Mr Henman, speaking to shoppers, knocking on doors and occasionally stopping for selfies with fans.
Earlier this week, Coogan was among 40 figures who signed a letter in the Guardian accusing Prime Minister Boris Johnson of offering "dog-whistle politics".
The letter also praised Labour leader Jeremy Corbyn's "transformative plan that prioritises the needs of people and the planet over private profit".
The signatories included author Noam Chomsky, film director Ken Loach, the band Massive Attack, poet Michael Rosen, and rapper Kane "Kano" Robinson.
They added: "In the UK, Labour's election manifesto under Jeremy Corbyn's leadership offers a transformative plan that prioritises the needs of people and the planet over private profit and the vested interests of a few.
"Boris Johnson's Conservatives have nothing to offer but the prospect of an ever more unequal and divided society, and dog-whistle politics."