Sen. Chuck Grassley, R-Iowa, raised concerns this week about an amendment to the Republican healthcare plan backed by Senate conservatives, saying he would not vote for a bill that would offer inadequate protections for people with pre-existing illnesses.

The amendment, proposed by Sen. Ted Cruz, R-Texas, would allow health insurance companies to sell plans that aren't Obamacare-compliant as long as they sell at least one plan that provides the full range of medical benefits required under Obamacare, including maternity coverage and addiction treatment. Supporters, including influential outside conservative groups, say the proposal would allow healthier people to buy plans according to their liking with lower premiums, while critics say that people who need more care, including those with pre-existing illnesses, would find their options for health insurance to be prohibitively expensive.

Grassley raised concerns about that possibility.

"There's a real feeling that that's subterfuge to get around pre-existing conditions," Grassley told Iowa Public Radio in a story that was published Wednesday. "If it is subterfuge and it has the effect of annihilating the pre-existing condition requirement that we have in the existing bill, then obviously I would object to that."

Republicans have struggled to assemble a healthcare bill that would repeal and replace portions of Obamacare. They are working to keep promises to guarantee coverage for people with pre-existing conditions, while seeking ways to make sure that coverage doesn't become too expensive. A Congressional Budget Office analysis of the current healthcare bill's draft, the Better Care Reconciliation Act, projected that premiums would rise in the short term and that 22 million more people would be uninsured by 2026 than under Obamacare. CBO has not finished scoring the bill with the Cruz amendment.

At least 10 Republicans have publicly opposed the legislation as the draft stands. Conservatives want a bill that will do more to repeal Obamacare and to reduce premiums, while centrists are worried about the CBO's projection about uninsured numbers and about cuts to Medicaid.

The Cruz amendment has become a line in the sand for Senate conservatives, according to Republican aides. The Senate is looking at increasing stability funding in the bill to help states with their health insurance markets, and conservatives could accept the increase in funding if the amendment were included.

Republicans are trying to pass the bill through reconciliation, which requires 50 votes for passage, assuming a tie-breaking vote by Vice President Mike Pence. Because no Democrat will vote for the bill, Republicans can lose no more than two of their own members.