The number of Americans losing their Obamacare plans due to insurer exits is starting to add up.

Aetna is the latest insurer to announce it is pulling out of most of the insurance marketplaces set up under President Obama's healthcare law. UnitedHealthcare and Humana both said earlier this year they will withdraw from most of the exchanges due to higher-than-expected losses.

Individually, none of the insurers holds a major market share in the Obamacare exchanges. But the cumulative effect of all three pulling back could be more serious, hurting competition among plans, driving up premiums and making it harder for customers to find a plan they like.

"If it was just United that was pulling out, or just Humana, or just Aetna, it's not horrible," said Charles Gaba, who has successfully predicted Obamacare enrollment and runs the website

"But when you have three major ones pull out in the same year, combined, that's a lot," Gaba said.

It's hard, if not impossible, to calculate precisely how many people will see their Obamacare plans canceled next year, since neither insurers nor the Centers for Medicare and Medicaid Services have comprehensively reported insurers' market share by state or county.

But it could be about one in six Obamacare customers, or 17 percent, using enrollment numbers reported by insurers.

Aetna and UnitedHealthcare have 838,000 and 795,000 marketplace customers, respectively. Humana has 875,700 customers, although that includes customers in individual market plans off the exchanges.

Those customers total 2.5 million people, although some of them live in the handful of states where the insurers will keep selling plans next year. But they still comprise a significant portion of the 11 million Americans who have bought Obamacare plans in the marketplaces.

"I think it's sobering," said Gary Claxton, vice president of the nonpartisan Kaiser Family Foundation.

Aetna's recent pullout is likely to hit some states hard, as the company announced Monday it is pulling out of 11 of 15 states where it offers plans.

While some of the states are larger and can take the hit, such as Florida and Texas, others will see competition slide. For instance, South Carolina will have only two Obamacare insurers after Aetna leaves and both of the insurers (BlueChoice and Blue Cross/Blue Shield) are owned by Anthem, according to data from, a consumer insurance guide.

North Carolina's Obamacare marketplace also will be reduced to two competitors: Cigna and Blue Cross/Blue Shield. The exit also leaves Kentucky, which recently shuttered its state-run Obamacare exchange, with four insurers.

Some politicians are seizing on the pullout, saying it will hurt competition in their localities. Sen. John McCain, R-Ariz., pointed to the lack of competition Tuesday.

"It's especially disturbing that this move could negatively impact nearly 10,000 citizens enrolled in Obamacare in Pinal County, Ariz., where not a single health insurer has filed to offer federal exchange plans," he said.

McCain added that without any insurer, county residents would have no healthcare options to join Obamacare's marketplace.

But some advocates for the healthcare law are pushing back. John Holahan, a fellow with the left-leaning Urban Institute, said it's hard to say now whether the county will go without an Obamacare carrier as another insurer could still enter the market.

"You don't know yet that nobody else would come in," Holahan said. "I wouldn't be surprised if Arizona puts some pressure on a Medicaid plan [insurer] Meritus that has a big market share in Arizona."

He said that Aetna, Humana and UnitedHealth at times didn't price plans aggressively and don't have much market share.

"When they pull out of those, it is probably not going to be terribly damaging," Holahan said.

Department of Health and Human Services spokeswoman Marjorie Connolly told the Washington Examiner her agency is "working collaboratively with the Arizona Department of Insurance and remain confident that all Arizona residents will have access to coverage next year."

Others slammed Aetna's decision to withdraw from the marketplaces. Rep. Frank Pallone, the top Democrat on the House Energy and Commerce Committee, accused the company of retaliating against the Justice Department for trying to block its proposed merger with Humana.

"Aetna's decision to exit a majority of the [Affordable Care Act] marketplaces puts profits over people," he said.

Analysts say the key in the next enrollment period, which starts in November, is whether the Obama administration succeeds in getting more Americans to enroll in plans. If that happens, insurers would be drawn to the marketplaces, he said. But if enrollment stalls, insurers would be turned off from the exchanges.

"[Enrollment's] got to keep going forward for the insurers to be convinced this is a good market and that's what's going to matter," Claxton said.

The Obama administration is trying hard to convince the public that the marketplaces will still offer the uninsured good plan options at competitive pricing, despite the growing list of insurers opting out.

"We are confident that consumers will continue to have affordable options for coverage next year," Connolly said.