Norman Lamb and Tim Farron put aside their personal ambitions to launch a joint appeal to save the legislation, which the Government wants to replace with a British Bill of Rights.
Former health minister Mr Lamb said the Lib Dems blocked Mr Cameron from abolishing the legislation during the coalition’s term in office while Mr Farron said their plans now showed the Tories’ “true colours”.
Proposals to replace the Human Rights Act are expected to be contained in this week’s Queen’s Speech which will allow the Prime Minister to set out his plans for the first time as leader of a majority government.
The two rivals to replace Nick Clegg, the former deputy prime minister who quit in the walk of the Lib Dems’ disastrous general election results, added their voices to critics of the plan to scrap the Act.
Mr Lamb said: “Our human rights laws have already achieved so much and we must not let the Tories trample all over them.
“They have stopped the state spying on us, supported peaceful protest and guarded against slavery. They have helped rape victims, defended domestic violence victims and shielded press freedom.
“The Liberal Democrats blocked David Cameron from scrapping the Human Rights Act in Government and we must stop him again now.
“That is why we are urging everyone to get behind our campaign to stop the Tories riding roughshod over our freedoms and rights.”
Westmorland and Lonsdale MP Mr Farron said: “By drawing up plans to scrap the Human Rights Act and laying the groundwork to pull out of the European Convention on Human Rights - the Tories are revealing their true colours.
“It sends a clear message to the British people: the next five years will be nastier, less liberal and less compassionate that any Government in living memory
“These are fundamental rights we all have and they define how we treat our citizens and offer our citizens protection from the state.
“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.
“We blocked them before and we must do all we can to block them again.”