The letter -- from one of the most influential groups in domestic policy -- is the latest blow for a bill that has already seen opposition from many conservative leaders, media organizations and interest groups.
"This bill would weaken Medicare's fiscal sustainability, dramatically increase health care costs for Americans aged 50-64 and put at risk the health care of millions of children and adults with disabilities, and poor seniors who depend on the Medicaid program for long-term services and supports and other benefits," said the letter, signed by Joyce A. Rogers, a senior vice president at AARP.
The group represents millions of seniors across the country and has long opposed any policy it perceives as weakening Medicare, a government health care program exclusively available to the nation's elderly.
AARP was a crucial backer of the Affordable Care Act, popularly known as Obamacare, and has repeatedly defended the law over the years.
The AARP letter said Obamacare helped strengthen Medicare, and that a tax cut for higher income workers in the American Health Care Act "could hasten" the program's insolvency.
When he was on the campaign trail, President Donald Trump pledged to protect Medicare and other major entitlement programs. However, the AARP has said the bill would do the opposite. Still, Trump has backed the proposal from House leadership and called on the country to unite around it, including the conservative factions of his party fighting to bring the bill down.
Under Obamacare's individual mandate, people must purchase health insurance, including young, relatively healthy people, or pay a fine. The forced inclusion of more people in the market was an attempt to balance out insurance markets for older people and those with pre-existing conditions. However, insurance providers are allowed to offer insurance at different rates depending on age.
AARP said the changes in age rating as well as reductions to subsidies would "dramatically increase health care costs for 50-64 year olds" who purchase insurance through government exchanges.
The group also railed against a slew of proposed changes to Medicaid, a government program aimed at helping people with low incomes and disabilities get health care.
Finally, the letter praised Republicans for what the bill did not aim to repeal from Obamacare, for example, protections for persons with pre-existing conditions and allowing young people to stay on their parents' insurance plans until age 26.