(CBS NEWS) -- Two of the country's largest insurance providers called an amendment from Sen. Ted Cruz to the latest Republican health care bill "simply unworkable," as Republicans struggle to capture enough votes to pass the legislation.

The CEOs of America's Health Insurance Plans (AHIP) and Blue Cross Blue Shield Association wrote a letter to Senate Majority Leader Mitch McConnell (R-Kentucky) and Senate Minority Leader Chuck Schumer (D-New York) Friday, urging them to strike the Texas Republican's "Consumer Freedom Option" amendment, claiming it will cause costs to skyrocket for those with "significant medical needs" and limit insurance options for people who buy individual plans.

Cruz's amendment would allow insurers to offer bare bones-style plans with fewer benefits, as long as those insurers also offer more robust plans required under the Affordable Care Act, or Obamacare.

"As the U.S. Senate considers the Better Care Reconciliation Act, we are writing to urge you to strike the 'Consumer Freedom Option' from the bill," the letter reads. "It is simply unworkable in any form and would undermine protections for those with pre-existing medical conditions, increase premiums and lead to widespread terminations of coverage for people currently enrolled in the individual market."

The Cruz amendment, offered to placate conservatives, allows insurers to provide plans with fewer benefits that don't meet the necessary list of "essential health benefits" under current law. The purpose of the amendment is to give people who don't need or want coverage for things like maternity care or mental health the option to buy a cheaper, bare-bones plan. But that scenario, critics and the insurance groups argue, leaves sick people in sicker, costlier risk pools, meaning higher premiums.

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