NEW MILFORD — Competing owners of big-box retail shopping centers are blasting a plan to rezone 39.5 acres on Route 7 that could pave the way for a Walmart Supercenter.

Landowners argue the move, which they say could create up to 600,000-square-feet of retail space anchored by a new Walmart, would harm existing businesses and the viability of other shopping centers in New Milford.

Part of the controversy around the proposal is the secrecy around it. Town Councilor Paul Szymanski is representing the owner of the property behind the Windmill Diner on Route 7, but has refused to comment on whether Walmart or someone else has plans for it.

Willing Biddle, whose company owns the shopping plaza where Walmart is now, says he’s sure the zoning amendment proposal stems from Walmart corporate. Biddle said Walmart has told him it wants a larger space in the area, one that could house a Supercenter.

Kristen Gizzi, executive director of Litchfield Crossings, which is another large retail property owner on Route 7, says she doesn’t see the need for more retail space in town.

Litchfield Crossings, whose tenants include Kohl’s and HomeGoods, has ready-to-rent storefronts and already-approved plots of land for retailers interested in moving in, Gizzi said. But the company has had a hard time finding takers.

“It’s already very challenging finding tenants,” she said. “I don’t think anyone wants to see an over-saturation of retail. That doesn’t benefit anybody.”

The issue has also become a partisan fight.

Szymanski, a Republican, has been sharply criticized by Mayor David Gronbach, a Democrat, for not revealing whether Walmart is behind the proposal. And Gronbach has also criticized Republican Town Councilor Tom Esposito for writing a letter in support of the zone change.

“I implore Councilman Szymanski and Esposito to tell the community what the plans are for this property,” Gronbach said, adding that the planning commissioners who voted last week to move the project forward to the zoning commission “voted to approve a change without knowing the ultimate use or considering the effect on the town.”

“That is the opposite of planning.” Gronbach said. “For two town council members to endorse the change without disclosing the ultimate use is the opposite of transparency.”

The planning commission’s vote was split along party lines - with three Republican members backing the change and two Democratic members opposing it.

Szymanski, president of Arthur H. Howland & Associates, put the amendment forward on behalf of the executors of an estate that owns the property.

He told the planning commission that there is a potential buyer for the land, but refused to identify the company.

Wal-Mart has not responded to repeated requests for comment, but its 2016 annual report tells investors it’s looking for “a moderation of Supercenter growth in the U.S.”

Connecticut has only 12 Supercenters, while nearby states New York and New Jersey have 78 and 26, respectively, according to Walmart.

In an interview, Gronbach described the proposed zone-change as “pulling the rug out” on local businesses and property owners.

He outlined a scenario in which Wal-Mart leaves Biddle’s plaza and hurts nearby businesses.

Biddle agreed that Walmart leaving his plaza would hurt stores there, but said he thinks the net loss for businesses across town would be much worse.

If a Walmart Supercenter were to open on 39.5 acres of land in New Milford, Biddle argues, several businesses from around the area would flock to leftover acreage from the project. A Walmart on that land, doesn’t just mean a Walmart with groceries will be there competing with other grocery stores, he said. It also means a new shopping center, soon full of stores that have left their current homes, leaving behind vacant storefronts elsewhere.

If the zoning amendment goes through, Biddle said he would be tempted to sell his land and get out of town.; 203-731-3411; @bglytton