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.

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"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.

"The Consumer Freedom Option establishes a 'single risk pool' in name only," the letter says. "In fact, it creates two systems of insurance for healthy and sick people."

"As healthy people move to the less-regulated plans, those with significant medical needs will have no choice but to stay in the comprehensive plans, and premiums with skyrocket," the letter added.

The letter could be dismissed by Republicans, however, as the current head of AHIP is Marilyn Tavenner, who helped implement Obamacare as the head of the Center for Medicare and Medicaid Services.

The Cruz amendment, the insurers argue, would "especially" hit families who earn too much to qualify for a tax credit but can't afford the higher premiums for more robust plans. Under Obamacare, families and individuals earning up to 400 percent of the poverty line are eligible for tax credits.

The bill as-is will also result in "far fewer, if any" options for people who buy their plans from the individual market, the insurers claimed, meaning "millions of more individuals will be uninsured."

McConnell has said he wants to vote on the health care proposal as early as next week, but at least two Republicans senators already oppose the bill. If McConnell loses one more vote -- no Democrats are currently in favor of the bill -- it won't pass. This is the Senate's second attempt at devising legislation. The first attempt failed in June, as Republicans lacked the votes to pass their bill.

The Congressional Budget Office has yet to score the latest Senate bill, but the bill the House passed earlier this year would leave 23 million more people uninsured, and the previous Senate bill would leave 22 million more people uninsured.