WASHINGTON, D.C. - Ohio Gov. John Kasich on Tuesday joined a bipartisan group of governors who formally declared opposition to the latest U.S. Senate plan to repeal and replace the Affordable Care Act.

Kasich joined nine other governors including Colorado Democrat John Hickenlooper in a letter that asked Senate Majority Leader Mitch McConnell and Democratic Leader Chuck Schumer to not consider the controversial proposal.

Their letter said the Senate's Health, Education, Labor and Pensions Committee has held hearings on ways "to make individual health insurance more stable and affordable," and that committee's efforts should be supported rather than the Graham-Cassidy-Heller-Johnson amendment that was advanced last week and could be voted by the end of the month by the Senate.

Once again, bipartisan governors have joined together to send a strong message: the only way forward on health care is if we work together. pic.twitter.com/9WuJkrKC3P — John Kasich (@JohnKasich) September 19, 2017

The proposal that Kasich opposes was advanced by GOP senators from South Carolina, Louisiana, Nevada and Wisconsin, who said it would repeal the structure and architecture of Obamacare and replace it with a yearly block grant to states that would help individuals pay for health care.

Schumer calls the plan "an evil bill" that would cost 30 million people their insurance, gut Medicaid and eliminate pre-existing conditions protections.

McConnell - who is trying to round up the needed 50 Republican votes to pass a health care reform plan - called the Graham-Cassidy proposal "an intriguing idea" in a Tuesday speech, and warned that the opportunity to "move beyond the failures of Obamacare" ... "may well pass us by if we don't act soon."

The opposition from the bipartisan governors will complicate McConnell's efforts.

Calling your representatives will impact the outcome of Cassidy-Graham. We've defended healthcare once & we can do it again: (202) 224-3121 https://t.co/uKfrp3i144 — Sherrod Brown (@SherrodBrown) September 18, 2017

Their letter said any solution should "control costs, stabilize the market, and positively impact coverage and care of millions of Americans, including many who are dealing with mental illness, chronic health problems and drug addiction."

"We ask you to support bipartisan efforts to bring stability and affordability to our insurance markets," it said. "We look forward to continuing to work with you to improve the American health care system."

Ohio Republican Sen. Rob Portman told reporters on Tuesday that he's still reviewing the bill and its impact on Ohio and will decide soon whether to support it.

He said he likes that the bill would give states flexibility to meet their own needs, and observed that Kasich would be out of office when the bill becomes effective in 2019.

"Our needs are different than other states," Portman said, adding that it would give Ohio's legislature and governor the ability to design programs to address state-specific problems, like opioid addiction.

To #Congress: Graham-Cassidy would result in millions losing coverage, destabilize insurance markets, decrease access to affordable care. pic.twitter.com/p0A1lJf4cx — AMA (@AmerMedicalAssn) September 19, 2017

Last month, Kasich and Hickenlooper led a group of eight governors in setting forth a list of proposals to fix problems in the Affordable Care Act's insurance markets. Their ideas included federal funding of cost-sharing payments to insurance companies, creating "stability plans" for states, and giving insurance companies breaks for offering plans in underserved counties where only one carrier has agreed to offer ACA policies.

The governors of Nevada, Pennsylvania, Alaska, Virginia, Louisiana and Montana joined Kasich and Hickenlooper in last month's proposals and Tuesday's letter. The governors of Massachusetts and Vermont signed onto the letter alone.