House Majority Leader Kevin McCarthy (R-Calif.) on Friday pushed back on conservatives’ calls to change the GOP’s bill repealing and replacing ObamaCare.

Conservatives have said the American Health Care Act should freeze ObamaCare’s Medicaid expansion in early 2018, rather than 2020. But McCarthy threw cold water on the idea, saying it would be “very difficult to do.”

House Energy and Commerce Committee Chairman Greg Walden (R-Ore.) added that he did not want to create a “gap” in coverage, noting that the current date tries to strike a balance among different states, who vary in ways such as how often their state legislatures meet.

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The pushback comes as conservative objections are threatening passage of the legislation, and conservatives are looking for changes.

But leadership is pushing forward with the current plan, emphasizing that other ideas could come in a different “phase” of the plan as separate legislation that could be voted on even the same week as the main bill.

“If we’re going through three different phases, in that third phase, we can bring up and pass that piece of legislation that other members can feel that it gives them greater support to pass the bill,” McCarthy said.