Between 6 million and 10 million people would lose health insurance coverage if a Republican proposal to replace the Affordable Care Act becomes law, a new report estimates.

The top end of those losses, or 10 million people, is equal to half of the 20 million or so people who have gained coverage in the past seven years under Obamacare.

The sobering estimate came from S&P Global Ratings on Tuesday, less than a full day after House GOP leaders released their plan for gutting key elements of the ACA, and replacing it with new rules.

S&P Global Ratings said it expected that if the plan were approved, there would be a decline in enrollment in the individual health insurance plan market of between 2 million and 4 million people.

There also would be a decline of between 4 million and 6 million people in the nation's Medicaid system after 2020 to 2024, according to the report.

But profitability among U.S. insurers "will likely improve," the company said, pointing to the fact that the replacement plan "can result in an improved risk pool in the individual market."

The report said that the replacement plan would likely increase the affordability of individual insurance plans for younger adults, and reduce the affordability of those plans "for the older population."

S&P Global Ratings also said it expected there would be a "large difference between states both in terms of insured rates," that is the percentage of people with health coverage, "and benefits covered by insurance plans."