The Scottish government is being urged to step in over the future of up to 12 Bield care homes.
The firm has said financial constraints means it has to look differently at how the business operates.
Unison has warned that the closures would be a “disaster”, with about 140 elderly people losing their homes.
Bield said it was in advanced talks to secure a new provider for four of the care homes in Edinburgh, Fife and Jedburgh.
Unison also said that up to 200 care workers were facing compulsory redundancy.
The union will back calls for government intervention at a public meeting due to take place for family and friends of Bield residents.
At least eight of the care homes are set to be closed by the spring.
Many of those currently living in the homes need special care such as feeding assistance or 24-hour care.
The homes are spread across Edinburgh, Falkirk, Glasgow, the Borders, South Lanarkshire and West Lothian.
Families have said the closures are tantamount to eviction and have warned of the detrimental effects of moving frail, vulnerable people.
Unison’s Scottish organiser John Gallacher said: “We are disappointed that Scottish government ministers seem content to stand by and let the closures happen, when the lives of vulnerable elderly people are at risk, and the jobs of essential care workers in many Scottish communities are being jettisoned.
“If it goes ahead it will be a disaster.”
He said that new integrated joint boards “should be subjected to proper political and public scrutiny and held accountable for allowing such damaging developments to take place within ridiculously-short timescales.”
A spokesperson for Bield said that the care homes had been loss-making for a number of years.
“Despite many attempts to research and consider alternative models to address the challenges, Bield have regrettably not been able to find a solution that ensures financial viability while maintaining high standards of care.”
Although Bield is withdrawing from the care home market, it insists it has developed a five-year strategy to create a “new-look organisation” which will continue to provide valuable services for older people.
A Scottish government spokeswoman said: “We recognise how unsettling this decision has been for all the residents, families and employees affected.
“We are working with Bield and all relevant agencies to ensure that residents’ care needs continue to be met.
“The Cabinet Secretary for Health and Sport is due to meet the chief executive of Bield Housing and Care later this month to discuss progress.”