ROCKLAND, Maine — Sides are lining up in advance of a planned Monday night vote by City Council to impose a six-month moratorium on electrical generation plants, a move that threatens a proposed natural gas power plant.

Supporters of the moratorium say the time is needed for the city to develop ordinances that would protect the health and safety of Rockland residents. Opponents, however, say it likely will kill any chance the city has of getting the power plant, its jobs and tax dollars.

The opponents also warn a moratorium will send an anti-business message beyond the city’s borders.

Two groups stepped forward Friday to voice concern about the moratorium: the local Chamber of Commerce and the city’s firefighters. Both groups sent letters to the city decrying the proposal.

The moratorium was suggested in response to Rockland Energy Center’s announcement that it is developing plans to build a 35 megawatt power plant somewhere in Rockland. Company partner Evan Coleman told the council Dec. 7 that Rockland Energy, which initially sought to acquire and build on city land — where City Hall and the public services garage are located — had decided to shift to private property because it would create fewer hurdles. If the city had reached a purchase and sales agreement with Rockland Energy, the deal would require voter approval at referendum.

At that same meeting, Coleman told councilors he planned to return before them with more details and plans in early February.

In the letter sent to the council later Friday, the Penobscot Bay Regional Chamber of Commerce board wrote, “We struggle to understand the rush for a moratorium at this time, when we are so close to receiving greater detail that will make a determination of the merits of this project much clearer. We encourage you to give Rockland Energy Center an opportunity to provide you with more detailed plans, so that your decisions can be based on facts from a proposal, rather than speculation and fear of the unknown.”

Chamber officials indicated they agreed a decision whether to support the proposal from Rockland Energy Center was premature, given the lack of details on revisions to their plans.

“However, a moratorium serves to end the information gathering process on this project, and does not send a positive message about the City of Rockland’s openness to economic development proposals for the city,” the board stated.

The Rockland Professional Firefighters said in their letter to the council that city officials need only to look out a window at City Hall to see an example of a development project that ended up moving elsewhere. Wal-Mart wanted to build a supercenter in Rockland but because of local opposition instead located the new store in neighboring Thomaston within a few hundred yards of the town line.

“We lost out on an increase in tax revenue from a large expansion and gained all the headaches that come with increased traffic and the abuse of our roadways. If you are taking note of the new ‘Super Center’ then also picture an energy plant right beside it,” the firefighters stated in their letter.

Meanwhile, another developer appears to be reviving a separate yet similar energy plant project first introduced in Thomaston about four years ago. A company called Self-Gen Inc. proposed in December 2011 building a 25-megawatt natural gas co-generation plant adjacent to Dragon Products in Thomaston that would generate electricity and steam.

Self-Gen President Paul Aubrey of Scarborough said in May, after Rockland Energy expressed interest in the Rockland property, that his company was reconsidering its proposal.

“We spent several years developing the models and strategies but could not get commitments from the private or public sectors. We will look to reconnect with the original stakeholders now that we have a source of business development funding,” Aubrey said earlier this year.

Aubrey said excess electricity from the cogeneration plant would have gone to a municipal energy account for low-cost electricity to the Rockland area, not to the grid for retail sales.

Thomaston Town Manager Valmore Blastow said Thursday that Rockland Energy has not approached the town but pointed out that the proposal to extend natural gas to Rockland began with the Self-Gen proposal that would have offered numerous benefits.

Thomaston has pursued a pro-growth policy over the past decade that has seen significant retail development along Route 1 near the Rockland line. That development accelerated after Thomaston residents rejected a moratorium on big-box stores. Thomaston has seen its tax rate decline over the past few years.

The Rockland firefighters said a project such as the energy plant proposed for the city would offer property tax relief. They said the city has cut costs and delayed improvement projects over the years because of budget issues.

“We have lost entire city departments because the only costs left to cut are city services. Without an increase in tax revenue we will be forced to make even more difficult decisions, ones that could involve greater loss in services,” the firefighters stated.

The moratorium to be voted on Monday would last for six months and cover electrical power generation facilities having a capacity in excess of 10 megawatts. The proposed moratorium directs the city’s energy advisory committee to work with the planning board and instructs the planning board to get technical experts to help come up with recommended ordinances to regulate energy plants. The ordinances also would regulate the installation of natural gas lines in the city.

Councilors are split on the issue, though three of the five council members have expressed support for the measure. The moratorium was put forward by William Jillson, who was elected last month to the municipal legislative board.

Councilor William Clayton said Friday he has not made up his mind on the energy plant because a final plan has not been presented. He said, however, that by a wide margin he has heard from more people in favor of the plant than opposed.

Clayton said a moratorium could generate lawsuits. He questioned whether it was being proposed to give time for new regulations or simply to prevent the energy plant from being built anywhere in Rockland.

Councilor Larry Pritchett said he has seen dozens of communities enact moratoriums to allow officials time to adopt new ordinances. He said the type of plant being proposed was likely not even contemplated when Rockland created its zoning laws.

He added he strongly doubted a moratorium of such short duration, that would address major development, would create a legal risk for the city.

Mayor Louise MacLellan-Ruf said the moratorium was sending a message to the business community that doing business in Rockland was unpredictable.

“A moratorium feels like a knee-jerk reaction that just does not make logical sense,” she said.

“Rockland needs to be predictable. Moratoria are not meant to be used in a willy nilly fashion. Moratoria are not decided on whether someone likes a person or does not. Nor should a moratorium be selected as an option because some may not like a business proposal,” the mayor said.