Senate Republican staffers estimated Thursday that the annual cost to taxpayers of legalizing illegal immigrants could be in the billions, if they use their status to apply for federal benefits from Medicaid and ObamaCare. The claim, though, was challenged by Florida Sen. Marco Rubio, a key Republican engaged in immigration bill talks.

Republicans on the Senate Budget Committee — responding to an emerging immigration proposal that includes a path to citizenship for millions of illegal immigrants — had estimated that the cost could total up to $40 billion in 2022, “just for Medicaid and ObamaCare.”

“The net costs would be enormous and only increase once citizenship is granted,” the office of Alabama Sen. Jeff Sessions, top Republican on the committee, said in a statement Thursday. The estimates were crunched by budget committee Republican staff.

Payments from Medicaid and subsidies from the federal health care law represent just a fraction of federal government benefits that a green-card holder or U.S. citizen can apply for.

But Rubio spokesman Alex Conant disputed the numbers, saying in a statement late Thursday that it’s simply too early to provide cost estimates.

“Since we don’t yet have a legislative proposal, it’s not possible to come up with anything resembling an accurate calculation about the potential fiscal impact of bipartisan immigration reform,” he said.