The predictions come as GOP leaders face difficulty getting 50 Republican senators to support the controversial repeal bill pending in the Senate.
In one report, S&P Global Ratings said the bill would add "flexibility" for individual states by offering them block grants of federal funding to craft their own health insurance systems.
But the lower overall federal funding that they would get compared with what many of them currently get through Obamacare "would hurt the economy, states, and health insurers," that report said.
The analysis also said the bill would initially lead to fewer people with health insurance who earn between 133 percent and 400 percent of the federal poverty line, an income band that encompasses most people eligible for Obamacare subsidies to help pay for health plan premiums.
And over time, there would be fewer people insured by Medicaid, the joint federal-state program that covers poor people, "as some states are unable to maintain current eligibility levels" because they will get less federal funding under Graham-Cassidy.
In another report, S&P Global Ratings said the bill "represents nothing short of an overhaul" to the fiscal relationship between federal government and individual states, which could increase pressure on states' budgets.
The company noted that funding by the federal government for Medicaid accounts for more than half of all federal funding to states, at a whopping $330 billion in 2016.
Federal Medicaid funding functions "as a fiscal stabilizer for state finances," because that federal money increases during economic downturns.
But the Graham-Cassidy bill "would upend Medicaid's status" as a program in which, currently, federal funding increases as a state's costs from the program rise. Under the bill, that increased federal funding would be capped, meaning states would get less money than they currently are projected to receive through Medicaid.
That would "result in more acute financial pressure on the states in economic downturns," according to the analysis.