Tory plans to shake-up funding for social care could see nine out of 10 English homes eligible to be put on the market to cover treatment costs, the Liberal Democrats have said.
Lib Dem leader Tim Farron has called for a national cross-party campaign to combat the so-called Conservative "dementia tax".
The Tory manifesto commits the party to get people to pay for their own care if they have combined savings and property valued at more than £100,000.
If they wish to keep their home, payment can be deferred until after they die when it will be deducted from their estate.
Lib Dem research shows that, overall, 90% of homes in England would be liable to be sold under such conditions, and in the poorest 10% of local authority areas it would be 50%.
The party said that only one of the 356 dwellings sold in Prime Minister Theresa May's local authority area this year would be exempt from such an initiative.
In an open letter to health and older people's charities, Mr Farron claimed such a change of policy would have a damaging impact.
He said: "The measure of a government is how they treat the most vulnerable in our society. I don't think that the Conservatives are unaware of the impact of their plans but they chose to ignore the human cost.
"Every elderly person that needs care should receive it in the best place for them and not be fearful of those mounting, limitless costs.
"I am determined that we ensure that Theresa May drops the so-called 'dementia tax' and implements a cap on the cost of care.
"Caring for our elderly must be above party politics and that is why I want to urge anyone who opposes the Conservatives' plans to come together to stop it.
"We must resist this plan and challenge would-be Conservative MPs to reject and stop it.
"As a first step, I am urging people to sign up at dementiatax.org.uk to help. I hope you will urge the supporters of your organisation to do the same.
"Together we can stop the so-called 'dementia tax'."