 -- Physician and hospital groups are voicing opposition to the new health care bill, the American Health Care Act, over concerns that many patients could lose health coverage.

Those opposed to the bill include the American Hospital Association, the American Medical Association and the American Academy of Pediatrics.

In a letter to congressional leaders, Dr. James Madara, the CEO of the AMA, said the bill could harm vulnerable patients in its current form, especially those covered by the expansion of Medicaid under the Affordable Care Act, which provides federal matching funds for states who choose to opt-in and offer Medicaid to adults who are up to 138 percent above the poverty line.

"While we agree that there are problems with the ACA that must be addressed, we cannot support the AHCA as drafted because of the expected decline in health insurance coverage and the potential harm it would cause to vulnerable patient populations," Madara wrote on behalf of the AMA, the largest association of physicians and medical students in the U.S.

He cited concerns about the plan outlined in the AHCA to stop Medicaid expansion in 2020. Under the AHCA, states that offer the Medicaid expansion would continue receiving federal funds for those already enrolled, but would not receive federal funds as of 2020 for new candidates who only qualify for Medicaid under expansion rules.

"Medicaid expansion has proven highly successful in providing coverage for lower income individuals," Madara said.

The American Hospital Association also sent a letter voicing "significant concerns" about the current bill. Specifically, it believes the bill could lead to "tremendous instability" for people seeking affordable health insurance.

"Absent Congressional Budget Office analysis, our assessment of this legislation as currently drafted is that it is likely to result in a substantial reduction in the number of Americans able to buy affordable health insurance or maintain coverage under the Medicaid program," officials from the American Hospital Association said in the letter.

The American Academy of Pediatrics, an organization of 66,000 physicians specializing in medical care for children, teens and young adults, opposed the bill in a letter to members of Congress, saying it would undo recent gains in health insurance coverage for children. The group estimated that 95 percent of children are now insured, including those covered through Medicaid, the Children's Health Insurance Program and the Affordable Care Act.

The AAP opposed Medicaid changes under the American Health Care Act, such as moving away from an entitlement program, with states receiving open-ended federal funds for Medicaid recipients, to block grants, with states receiving per person allotments that cannot exceed designated totals.

"Medicaid has been a crucial source of health care coverage for children for over 50 years," the group said, reporting that 36 million children are currently covered via Medicaid. "Per capita caps would degrade the quality of care offered in the Medicaid program and would hinder the ability of states to respond to public health crises and other fluctuations in health care costs and the need for services."