Ron Pollack, who heads the healthcare advocacy group Families USA, said he is worried proposed spending caps and other budget cuts to Medicaid "would devastate seniors and people with disabilities."
Pollack also warned it could create "havoc" in families by forcing people to give up jobs to care for a parent, spouse or other family member.
Pollack's group, which backed President Barack Obama's healthcare overhaul, said Medicaid is the biggest payer of long-term nursing home care, which costs an average $74,800 per person last year.
Costs in some states are much higher and many people who pay for long-term care exhaust their financial resources and end up in Medicaid, said a study released by Families USA.
Pollack said the group released the report out of concern that budget hawks are focusing on Medicaid because spending cuts on Social Security retirement and Medicare elderly healthcare programs will lose votes in next year's presidential and congressional elections.
"Seniors vote and as a result it is politically much more difficult to make changes that are going to have an adverse impact on the folks who depend on those programs," Pollack said.
"Quite frankly, I don't think congressional Republicans believe that their votes are found in those who participate in the Medicaid program," he said.
Poor people, often minorities, generally support Democrats.
Republicans argue that Medicaid is becoming too big a strain on states that pay for most of the program's costs -- with help from the federal government.
Medicaid takes up a third of many states' budgets, and for nearly all, it is the fastest growing spending area.
Republicans have proposed giving states more control over the program through block grants and legislation that would allow states to opt out of healthcare law requirements to maintain Medicaid coverage levels as the new healthcare law is implemented.
The law, when it goes fully into effect in 2014, opens up the Medicaid program for millions of uninsured low income and poor people who currently do not qualify.