A key Republican swing vote on the Senate health care bill said Monday that he opposed the current plan because it did not roll back Obamacare’s protections for people with preexisting conditions, a demand that will likely make it difficult to win his vote.

Sen. Ron Johnson (R-WI) wrote in a New York Times op-ed that the current bill from Senate Majority Leader Mitch McConnell (R-KY) “leaves in place the pre-existing-condition rules that drive up the cost of insurance for everyone.”

Instead, we should return more flexibility to states, to give individuals the freedom and choice to buy plans they want without Obamacare’s “reforms.” And we should look to improve successful models for protecting individuals with pre-existing conditions, models underway prior to Obamacare, such as those in Maine and Wisconsin. Only then can the market begin to rein in the underlying cost of health care itself and reduce the cost of taxpayer subsidies.

The current Senate bill preserves much of Obamacare’s infrastructure, though it scales back its financial assistance for private insurance and gives states more leeway to roll back the law’s insurance regulations. It also ends Obamacare’s Medicaid expansion and puts a federal spending cap on the entire program.

But the Republican plan does maintain Obamacare’s requirement that insurers cover every American regardless of health status, and the prohibition on plans charging sick people more than healthy people.

Those are among the law’s most popular provisions, and a number of moderate senators might blanch if the Senate bill unwound them. There is also an open question of whether Republicans can undo insurance regulations under the complex procedural rules they are using to pass the bill with only 50 votes.

Johnson has wavered on the emerging GOP plan several times in recent weeks, first saying Republicans should start over with a bipartisan solution, then fretting over the truncated timeline in which Republicans are bringing up the health care bill, and now aligning himself with the chamber’s most conservative members.

If Johnson — along with Sens. Rand Paul, Mike Lee, and Ted Cruz — hold firm in their demand that the GOP bill roll back Obamacare’s protections for preexisting conditions, it could prove fatal. McConnell can lose only two of the 52 Senate Republicans.