As many as 90 percent of WellPoint customers have paid their first premium by its due date, according to testimony the company prepared for a congressional hearing Wednesday. For Aetna, the payment is in the "low to mid-80 percent range," the company said in its own testimony. Health Care Service Corp., which operates Blue Cross Blue Shield plans in five states including Texas, said that number is at least 83 percent.

Making the first monthly payment is the last step to confirm enrollment in plans sold under the Patient Protection and Affordable Care Act, and Republicans have made the question of how many paid a line of attack on the law.

"What you have here is very solid first year enrollment, no matter how you slice it," Dan Mendelson, president of Avalere Health, a Washington consulting firm, said in a phone interview. "This thing is, at this point, well entrenched."