Nearly a third of U.S. counties will be left with just one insurance option next year on the ObamaCare exchanges, according to a new analysis fueling warnings about the impact of the insurance company exodus from markets across the country.

The Kaiser Family Foundation study found residents in Pinal County, Ariz., are even at risk of having no insurance options on the exchanges, which provide subsidized plans.

Republicans seized on the report Monday to claim that the health care overhaul is not providing the choices promised by President Obama and others.

"The president repeatedly promised that his health care law would provide more choices, ‘bend the cost curve,’ and allow Americans to keep the plans they liked and could afford. He failed to live up to those promises, and families are paying the price," Sen. Roy Blunt, R-Mo., said in a statement, noting the majority of counties in Missouri could be left with just one insurance option on the exchanges.

The Kaiser Family Foundation study found that overall, 31 percent of counties will have just a single insurance option within the Affordable Care Act exchanges. That's up from 7 percent this year -- and underscores a problem many analysts have been warning about for years.

Further, about six in 10 counties could have two or fewer marketplace insurers in 2017, with the “bulk of the increase in single-insurer counties” the result of UnitedHealth Group’s exit, the study, released Sunday, reveals.

Indeed, six years after ObamaCare was signed into law, America’s major medical insurers, concerned about their own bottom line, have started to pull the plug on a variety of services and options available to consumers. Citing major losses, the top five insurers – Humana, Anthem, Aetna, UnitedHealth Group, and Blue Cross Blue Shield – have threatened to pull out of the exchanges and have selectively started to do so in many counties.

Near Phoenix, in Pinal County, Ariz., 400,000 residents are likely to have no insurer options on the marketplace next year. Both UnitedHealth and Blue Cross Blue Shield of Arizona plan to exit the area.

“As a rural area of our state in which 18 percent of the population lives below the poverty line, the impact will be felt particularly hard,” Sen. John McCain, R-Ariz., wrote in an opinion piece for FoxNews.com.

“My office has received countless other letters and phone calls from concerned Arizonans who have been left with fewer options, less access and decreased quality of care under ObamaCare,” McCain wrote. He added, “But this frustration is not unique to Arizona.”

Arizona had eight insurers operating in various parts of the state this year, but four are leaving entirely — Aetna, UnitedHealthcare, Humana and Health Choice. Two more, Blue Cross Blue Shield and Health Net, are scaling back their participation.

“Clearly this is a big concern for consumers,” Allen Gjersvig, director of navigator and enrollment services for the Arizona Alliance for Community Health Centers, told Kaiser.

He isn’t holding out much hope another insurer will step in, adding, “Things could change, but it’s not probable.”

When ObamaCare was first pitched to the public, the administration pushed the narrative that marketplaces would thrive and Americans who had been unable to afford medical coverage in the past would finally be able to do so.

While some aspects have been a success – 20.3 million Americans signed up for ObamaCare as of March 2016 – the unbalanced model of getting healthy younger people to foot the bill for older or sicker people who often require more coverage has presented a problem for ObamaCare and threatens its very existence.

The new report found that as insurers pull out, several states are now likely to have just a single insurance option across all counties.

“Given what is known at this time of entrants and exists, four additional states are likely to have a single marketplace insurer in all counties: Alabama, Alaska, Oklahoma and South Carolina, for a total of five states (including Wyoming, which already had one insurer in the states),” the Kaiser report said.

States with “significantly more single-insurer counties in 2017,” include Mississippi, Arizona, Florida, Missouri, North Carolina and Tennessee.

An analysis from the consulting firm Avalere found a similar problem, with the number of states with one marketplace insurer growing.

The health and sustainability of ObamaCare has also been a hot topic on the campaign trail. Democratic candidate Hillary Clinton has praised the ACA and said she hopes to improve on it.

Donald Trump has slammed the ACA’s instability. Stephen Miller, Trump’s national policy adviser, said ObamaCare is an example of Clinton’s “disastrously poor judgment.”

“Every policy she touches only produces more calamity,” Miller said in a written statement. “In this case, it means higher prices, fewer choices and less control over one’s most private medical decisions.”