AUGUSTA — Construction of a crucial forensic psychiatric unit in Augusta was dealt a blow by Democratic legislative leaders who, using an oft-overlooked oversight power, voted against Gov. Paul LePage’s administration’s plan on Wednesday.

LePage and the Department of Health and Human Services have been trying for months to start work on the building, which would house forensic patients — who are mostly people referred to the custody of Riverview Psychiatric Center by the courts — who don’t need acute care.

DHHS tried to move the project through the Legislature last year, but the bill was defeated amid concerns for advocates for the mentally ill and lawmakers who said the project was not fully vetted. That led the LePage administration to identify alternative, internal funding — outside of legislative oversight — and move forward with the project.

The proposed 21-bed, high-security forensic facility is crucial to the state’s efforts for Riverview to be recertified by the federal Centers for Medicaid Services and preserve $20 million in annual federal funding. The facility was decertified in 2013.

DHHS had assumed it could proceed with the $3 million to $5 million project without legislative approval, until Attorney General Janet Mills, a Democrat, notified the executive branch last month of a long-overlooked law that requires the Legislative Council to approve building projects in the Capitol Area, which is a designated development zone in Augusta.

The Legislative Council consists of 10 members of legislative leadership.

On Wednesday, with four members absent, Democrats and Republicans on the council deadlocked along party lines, 3-3, on the construction. Six votes are required for approval. In favor were the Republicans — Senate President Mike Thibodeau of Winterport, House Minority Leader Ken Fredette of Newport and Assistant House Minority Leader Ellie Espling of New Gloucester.

The Democrats — outgoing House Speaker Mark Eves of North Berwick and House Majority Leader Jeff McCabe of Skowhegan, and Assistant House Minority Leader Sara Gideon of Freeport, the incoming speaker of the House — voted against approving the project.

Gideon criticized DHHS officials for freezing the Legislature out of the project. She advocated for the approval to be delayed until the new Legislature is seated in January, when legislative committees can formally review it.

“It’s my own commitment and the commitment of other members of the Legislature to explore the best way to run a step-down facility,” said Gideon. “Why would we not take the time to expedite it through the committees with the questions that we need answered?”

Gideon said those questions ranged from how the construction and operation of the facility would be funded into the future to who would be responsible to its oversight. The LePage administration intends to have the facility run through a contract with a private vendor.

Ricker Hamilton, the department’s deputy commissioner of programs, said in a testy exchange with Gideon that the department has already devoted dozens of hours to answering questions from lawmakers.

“The people of the state of Maine need this council to vote today in support of this,” said Hamilton. “We can’t talk enough. The council needs to vote one way or the other.”

Hamilton said without council approval, the administration would likely be forced to seek another site, likely outside Augusta, where no legislative approval is needed.

Because of contracting issues, Hamilton said it would cost at least $1 million more than the current proposal, and possibly more depending on the site.

In a terse statement released after the vote, LePage said this is exactly what his administration now intends to do.

LePage pointed out that the “obscure” law that triggered Wednesday’s vote was ignored in some 30 other construction projects around the State House over the years, and he accused the opposing party of playing politics.

“Democrats have been using Riverview’s challenges as a tool to attack my administration for years,” said LePage. “All Democrats needed to do today was vote ‘yes,’ and Maine’s mental health system would be much better off, freeing up beds in Maine hospitals and delivering the appropriate level of care to all who need it.”