Currently, some 1,500 Constellation workers are based at the company's glassy headquarters on East Pratt Street and next door at the historic Candler Building on Market Place.
Meanwhile, the company's lease at the Candler Building, which provides about 187,000 square feet, runs through 2018.
The Downtown Partnership of Baltimore would like to see Exelon-Constellation remain at the two existing downtown buildings, said J. Kirby Fowler, the group's president.
Constellation's move could cause a domino effect, with tenants from other buildings also picking up stakes, Fowler said. "Creating a chunk of available office space … could have adverse ramifications for the rest of the office market," he said.
Manekin, the real estate broker, said the city's core downtown district would be better off if Exelon's new Baltimore quarters were consolidated into an existing building.
He noted that rents for office space downtown have dropped 20 percent to 25 percent in the past two to three years due to the "challenging economy and the abundance of vacant space."
"The drop in those rent rates coupled with the increase in building vacancy has lowered the overall economic value of commercial office buildings in downtown Baltimore, as well as the real estate taxes that these buildings generate," Manekin said.
But given Exelon's promise of a new building, the Downtown Partnership is advocating sites in the central business district, including the old McCormick plant, the former News American property and 1 Light St. All the locations are now used for parking.
Fowler also mentioned Baltimore City Community College's underutilized Bard Building on Lombard Street and Market Place, a block from the Inner Harbor, as a potential site.
BCCC spokesman Patrick Onley said that plans for the building were in the works but that no decision has been made on the type of development. Onley declined comment to comment further.
Fowler said the four business district sites are appealing because they "are in the middle of strong hotels and residential, retail and office community."
He also pointed to the "great water view."
In addition, given the business district's high vacancy rate, Fowler said it's important to "reinforce the historical core of downtown."
Baltimore Mayor Stephanie Rawlings-Blake continues to have discussions with Exelon and Constellation about their office needs, but her spokesman, Ryan O'Doherty, declined to provide details.
"Mayor Rawlings-Blake is committed to strengthening the downtown office and residential markets and would like to see additional private investment in the downtown central business district," O'Doherty said in a statement.
Outside the central business district, local brokers and others have identified the former Allied Signal chemical plant site between Harbor East and Fells Point as another attractive location for Exelon.
The Harbor Point project — led by bakery magnate and developer John Paterakis Sr. — would include apartments, condominiums, a hotel, a parking garage and shops. The Morgan Stanley building, which opened last year, is the first structure built on the 27-acre Harbor Point parcel.
Michael Beatty, president of Paterakis' H&S Properties Development Corp., did not return a call last week.
In recent years, Baltimore's business district has been expanding into the waterfront communities east of the harbor. Besides financial giants Legg Mason and Morgan Stanley, for-profit education companies Laureate Education and Educate Inc. are based in Harbor East, while First Mariner Bancorp has its headquarters in Canton.