"This is another blow to the casino industry here," Mazzeo said. "With mid-September the timing of the closing, it will have a devastating impact on the local economy."
Trump Plaza becomes the fourth casino to close or threaten to close in Atlantic City since January.
The Atlantic Club, which employed about 1,700, closed Jan. 13.
Revel Casino Hotel declared bankruptcy for the second time June 19 and is set to go on the auction block Aug. 6. Revel owners have said that if the casino does not fetch a buyer soon, the megacasino will close Sept. 1.
A week after Revel declared bankruptcy, Showboat Atlantic City announced it would close Aug. 31. Parent company Caesars Entertainment Inc. said this week that it was open to selling the casino to help save it from closure.
There has been speculation for more than a year that Trump Plaza was on the verge of closing. Like the Atlantic Club, Trump Plaza is one of the city's smallest and oldest gambling halls - it opened May 26, 1984 - and had difficulty competing with the bigger casinos in town and in nearby states, including Pennsylvania.
Bob McDevitt, president of Unite Here Local 54, the union that represents most casino workers, led a Boardwalk rally Wednesday to protest Showboat's planned closing. McDevitt, who labeled Showboat's closing "a criminal act" by Caesars Entertainment since the property was still profitable, could not be reached late Friday to comment on the latest casino to fall.
Whelan, a former Atlantic City mayor, expressed his displeasure Friday night.
"I go from depressed and sad to being angry," he said. "When these casinos close, people lose their jobs and their careers. It's a very sad situation."
Employees at Trump Plaza had not been notified Friday of the planned closure, but slot attendant Stan Jelesnianski, who said he has worked there for 21 years, said employees had been worried "for a long time."
"Business has been slow," he said.
According to May 2014 monthly revenues from the New Jersey Division of Gaming Enforcement, the latest monthly data available, Trump Plaza ranked last among the 11 casinos in total revenue, making $5.2 million.
Of that total, it generated about $4.6 million from slots, down 19.8 percent from May 2013. And it took in $660,666 from table games, a decrease of 45.6 percent from a year ago.
Its year-to-date total casino revenue of $21.9 million was down 26.7 percent from the same period a year ago.
Charles Pinkett, a Boardwalk rolling-chair operator, said the casino has seemed to be on life support for a while.
"The people have been talking about how there's no room service," he said.
Outside Trump Plaza, in the center of the Boardwalk on a busy summer evening, the casino's outdoor eatery was filled with customers and music could be heard from both nearby Kennedy Plaza and a beach bar.
Mark and Alice Aronson, dining outside, said they were surprised and saddened to hear another casino was on the way out. Alice Aronson, a local therapist, said she had clients from Showboat who were dealing with the pain of the likely layoffs.
"It's a shame," she said. "This is a good place. I heard Donald Trump is not involved anymore. I thought maybe he'd fight for it. It seems, one by one, they're not taking care of their employees."
Trump Plaza's closing would leaves one Trump-brand casino in Atlantic City - the Trump Taj Mahal, between Resorts Casino Atlantic City and the soon-to-close Showboat.