WASHINGTON― Lawmakers on both sides of the aisle are warning that a provision “defunding” Planned Parenthood could doom the GOP’s Obamacare replacement plan in the Senate because the language violates a key rule about what can be legislated with a simple majority vote.

The “Byrd rule,” named for the late Sen. Robert Byrd (D-W.Va.), requires that if a provision’s budgetary effect is “merely incidental” to a budget bill, it does not qualify for the fast-tracked reconciliation process in the Senate. This means that the legislation would have to meet the Senate’s usual 60-vote threshold to pass ― a much more challenging number for the 52 Senate Republicans to meet than a 51-vote majority.

The Congressional Budget Office confirmed in an analysis this week that the anti-abortion language in the American Health Care Act would only affect Planned Parenthood, even though the legislation does not specifically name the provider. Because this “defunding” provision appears to be politically motivated and would have little effect on the federal budget, it likely would not survive the Byrd test.

“The CBO report confirms what is already well known: Planned Parenthood is singled out for discrimination in Trumpcare for purely ideological reasons,” Sen. Jeanne Shaheen (D-N.H.) told The Huffington Post. “There’s very good reason to believe that this is contrary to requirements for expedited floor consideration in the Senate.”

Conservative Republican Mike Lee of Utah agrees with the latter point.

“This is a bad bill — one that will almost certainly be made worse once the Senate parliamentarian gives it the ‘Byrd bath’ required for it to go through the Senate with just 51 votes,” Lee wrote in an opinion piece for The Daily Signal on Monday. “The simple reality is that Republicans do not have the 60 votes in the Senate they need to pass a bill that repeals all of Obamacare and replace it with a new conservative health care system designed from scratch.”

This is a bad bill — one that will almost certainly be made worse once the Senate parliamentarian gives it the 'Byrd bath' required for it to go through the Senate with just 51 votes. Sen. Mike Lee (R-Utah)

There doesn’t appear to be a clear path forward for the bill. Senate Democrats will likely try to strip the Planned Parenthood provision from the legislation, which could bring moderate Republicans like Susan Collins of Maine and Lisa Murkowski of Alaska back on board. But if the Senate manages to pass an Obamacare replacement that maintains Planned Parenthood funding, many conservative lawmakers in the House would refuse to support it. The bill’s chances in the House are already dimming, with at least a dozen members of the House Freedom Caucus complaining that it’s not conservative enough.

Planned Parenthood, meanwhile, is engaged in an aggressive campaign to remind Congress of how popular it is with American voters. The family planning provider touted a new Kaiser poll on Wednesday showing that 75 percent of voters ― including a majority of Republicans ― favor continuing federal Medicaid funding to Planned Parenthood. While no federal dollars are used to pay for abortion, roughly 60 percent of Planned Parenthood’s patients rely on federal programs to reimburse the provider for health care services like birth control, pap smears and cancer screenings.

“Basic health care shouldn’t get caught up in congressional Republicans’ extreme agenda,” said Dawn Laguens, executive vice president of Planned Parenthood. “We’ve made tremendous gains in this country, including a 30-year low in unintended pregnancies and a historic low in teen pregnancies, thanks to expanded access to reproductive health care and birth control. Now is not the time to roll back that progress.”