One of the nation’s largest healthcare lobbying groups Monday announced its opposition to the Senate's ObamaCare repeal plan, warning that it could hurt the "most vulnerable citizens."

The American Medical Association (AMA) wrote a letter to Senate Majority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellDemocratic senator to party: 'A little message discipline wouldn't kill us' House to vote on resolution affirming peaceful transition of power Republican lawyers brush off Trump's election comments MORE (R-Ky.) and Minority Leader Charles Schumer Chuck SchumerPelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' 3 reasons why Biden is misreading the politics of court packing Cruz blocks amended resolution honoring Ginsburg over language about her dying wish MORE (D-N.Y.) warning against cuts to Medicaid and changes to ObamaCare's subsidies and regulations.

“Medicine has long operated under the precept of Primum non nocere, or ‘first, do no harm.’ The draft legislation violates that standard on many levels,” AMA Executive Vice President and CEO James Madara wrote in the letter

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"We sincerely hope that the Senate will take this opportunity to change the course of the current debate and work to fix problems with the current system," Madara added.

"We believe that Congress should be working to increase the number of Americans with access to quality, affordable health insurance instead of pursuing policies that have the opposite effect, and we renew our commitment to work with you in that endeavor."

The Senate GOP's bill, known as the Better Care Reconciliation Act, would end ObamaCare's Medicaid expansion by 2024 and cap how much the federal government spends on the Medicaid program.

It would also downsize the subsidies people get to purchase health insurance and make it easier for states to waive out of some ObamaCare regulations.

"It seems highly likely," Madara wrote, that the changes "will expose low and middle income patients to higher costs and greater difficulty in affording care."

The Senate bill would change Medicaid funding from an open-ended commitment from the federal government to states to a per capita cap, which essentially caps spending by how many enrollees a state has.

The bill slows down the growth of Medicaid spending, which would reduce the amount the federal government spends on the program by billions.

This change would "limit states' ability to address the healthcare needs of their most vulnerable citizens," Madara wrote.

"It would be a serious mistake to lock into place another arbitrary and unsustainable formula that will be extremely difficult and costly to fix," Madara said.

Madara also warned against language in the bill that would defund Planned Parenthood by blocking Medicaid reimbursements to the women's health provider for one year.

"We also continue to oppose Congressionally-mandated restrictions on where lower income women (and men) may receive otherwise covered health care services — in this case the prohibition on individuals using their Medicaid coverage at clinics operated by Planned Parenthood," Madara said.

"These provisions violate longstanding AMA policy on patients’ freedom to choose their providers and physicians’ freedom to practice in the setting of their choice."