Paige Winfield Cunningham and Robert King of the Washington Examiner chip away at the shaky foundation of the Affordable Care Act.

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.