Winston Churchill would abandon the Conservatives again and rejoin his old party in protest at David Cameron’s assault on the European convention on human rights if he were alive today, the Liberal Democrat president Tim Farron will say.

In his speech to the Lib Dem conference, Farron will accuse the prime minister of “trashing” Churchill’s record after the Tories said they would abandon the convention if a series of reforms are rejected.

The wartime prime minister was a strong supporter of the Council of Europe which drafted the convention after the second world war to ensure that universal human rights were enforced across the European continent.

Sir David Maxwell Fyfe, who was one of the prosecutors at the Nuremberg trials, was one of the original drafters of the convention. Churchill appointed Maxwell Fyfe as lord chancellor with the title Lord Kilmuir.

Farron will say: “The Tories’ major announcement was to scrap the human rights act, because, and I quote, ‘people get very frustrated with human rights’. So Mr Cameron, which of these do you find especially frustrating? The right to life, the ban on torture, protection against slavery, the right to a fair trial, respect for privacy, freedom of thought and religion, free speech and peaceful protest. These are not frustrations, these are integral to what it is to be British.

“They are the legacy of Winston Churchill. And seeing the Tories trashing his legacy, I am in no doubt that today he would once again be a Liberal.”

Churchill was initially a Conservative. But he spent 20 years as a Liberal before returning to the Tories in 1924 for the last 41 years of his life.

Farron will say that the two main parties have presented a depressing spectacle at their conferences over the last two weeks. He will say: “Have you ever seen a more miserable, uninspiring, tired, backward-looking sight than those two old parties meeting over the last two weeks? All the passion, conviction and vision of a convention of Breville sandwich-maker salespersons. Ed Miliband got a load of stick for only remembering half of his speech, but that’s still 50% more than the rest of us remembered.”

The Lib Dem president will also call on his party to revive the spirit of its past political giants to present an ambitious programme after the election. He will say: “I want active, ambitious, liberal government that identifies problems and seeks to fix them – taking Keynes and Beveridge into the 21st century.

“That means being prudent, wise and living within our means when it comes to revenue spending. But it also means opening the door as wide as we can on capital spending to transform Britain’s infrastructure. If we want to free our people, and defend them against the harsh winds of globalisation then we must literally build a better country.”