(CNN) Several powerful lobbying groups are lining up to oppose the latest Republican effort to repeal and replace Obamacare.

The influential American Medical Association, AARP and two major hospital groups all came out against a new amendment unveiled this week that would let states to weaken some of Obamacare's key protections for those with pre-existing conditions and for older enrollees.

Obamacare's protections for those with pre-existing conditions are among the law's most popular measures. Republicans say they are not eroding those provisions, but health care experts and lobbyists beg to differ.

"Although the MacArthur amendment states that the ban on pre-existing conditions remains intact, this assurance may be illusory as health status underwriting could effectively make coverage completely unaffordable to people with preexisting conditions," the AMA wrote in a letter Thursday to House Speaker Paul Ryan and Minority Leader Nancy Pelosi.

AARP slammed the amendment, saying it would put in place "backdoor cuts" to the protections and would create an "age tax."

"This harmful legislation still puts an Age Tax on older Americans and puts vulnerable populations at risk through a series of backdoor deals that attempts to shift responsibility to states," the influential lobbying group said in a statement Wednesday. "Older Americans need affordable health care services and prescriptions. This legislation still goes in the opposite direction, increasing insurance premiums for older Americans and not doing anything to lower drug costs."

AARP has been one of the most vocal opponents of the House GOP bill and has been particularly dismayed at the changes the House Freedom Caucus has gotten into the legislation. The group reiterated its promise to let its 38 million members know how their representatives voted on the legislation.

The hospital groups -- the American Hospital Association and America's Essential Hospitals -- both voiced concerns that the amendment would not lessen the number of people expected to be left uninsured under the Republican bill. The Congressional Budget Office analysis of the original legislation found that 24 million more people could be left without coverage by 2026.

"This latest version of the AHCA is not an improvement. It's simply bad policy that will cut a lifeline of health care for millions of Americans," said Bruce Siegel, CEO of America's Essential Hospitals, which represents safety net hospitals. "The amendment now being considered would threaten basic health care benefits and important consumer protections. Worse, these changes are expected to add to the already staggering total of 24 million additional uninsured people under the AHCA."