PORTLAND, Maine — Maine Gov. Paul LePage on Friday vetoed a bill to regulate sales of marijuana, citing concerns including how the Trump administration is going to treat the federal-state conflict in law.

His veto sets up a potential showdown when Maine lawmakers reconvene Monday.

“Until I clearly understand how the federal government intends to treat states that seek to legalize marijuana, I cannot in good conscience support any scheme in state law to implement expansion of legal marijuana in Maine,” the Republican governor told lawmakers.

Friday was the last day for LePage to veto bills to regulate the sale of marijuana, which was legalized by a statewide referendum vote in 2016.

The bill that sets rules and taxes on the retail sale of marijuana passed with a veto-proof, two-thirds majority in the Senate, but not in the House.

The Maine Marijuana Project called on lawmakers to override the governor’s veto and allow marijuana sales, likely in 2019.

“Governor LePage has made a mistake by vetoing this legislation. Instead of a regulated and controlled system of marijuana cultivation and sales, Maine will continue to support the unregulated market,” said David Boyer, political director of the marijuana advocacy group.

A group opposed to the bill, Smart Approaches to Marijuana, praised the governor for vetoing the “flawed bill.”

“This bill has gaping holes with respect to youth access and public safety. We now call on Maine lawmakers to do the right thing and sustain the veto,” said SAM Chairman Scott Gagnon.

Two weeks ago, the House and Senate approved a marijuana bill proposed by a bipartisan legislative panel. Members of the panel spent months rewriting the law to allow local communities to opt-in to recreational marijuana sales.

The bill also made other changes, including adding an excise tax to the existing 10 percent sales tax on recreational marijuana.

LePage has been critical of marijuana use, calling it a gateway drug, but he said in 2014 that the choice should be left up to voters.

On Friday, LePage said the Trump administration has not said whether it’ll take the same position as the Obama administration, which chose not to enforce federal law related to marijuana. He said he needs assurances from the Trump administration before establishing a new industry and regulations.

He said he had other concerns, including the bill’s failure to address compatibility with the existing medical marijuana program, the bill’s regulatory structure, and its “unrealistic timelines.”

LePage urged lawmakers to go back to the drawing board and avoid rushing the process. “If Maine is going to legalize and regulate marijuana, it is imperative that we do it right, he said.

Republican Sen. Roger Katz of Augusta said the governor’s criticism was “way more than day late and a dollar short” and said the governor should’ve engaged in the process.

“Like it or not, at this point, marijuana is legal. So that ship has sailed. As legislators, we can’t ignore the law. So now we have a choice: put a regulatory system in place or sustain the veto and fuel the fire of the expanding black market,” he said.