The proposal to build a Costco store in an apartment building on Manhattan’s Upper West Side is gaining an important supporter: Mayor Bloomberg.

At a press conference yesterday, Mr. Bloomberg said bringing the big-box warehouse chain to the city would help New Yorkers weather a difficult economic downturn. “Costco has a reputation of selling in bulk at very low prices, and given the economy today and the public’s desire to buy things in bulk and buy them cheaper, it seems to me we should welcome any store that wants to come here,” he said.

The big-box chain’s push for a store along the West Side Highway between 59th and 61st streets as part of a planned development by Extell Development Co. has sparked fierce opposition from local elected officials, business groups, and unions. The district’s council member, Gale Brewer, and Assembly member, Linda Rosenthal, have expressed concern that the store will increase traffic in the area and push out small businesses.

But Mr. Bloomberg said yesterday that efforts to prevent specific stores from opening in New York in favor of others are counterproductive and put the city on “a dangerous road” that could limit jobs and revenues at a time when taxpayers need help the most. Instead, he urged the city to bring in as many businesses as possible in order to boost the economy.

“If Costco or any other store or company wants to come here, we’d love to be a catalyst and help them find land, create the kind of jobs we need in this city, and give our citizens the opportunity to have a broader range of goods to buy at better prices,” Mr. Bloomberg said. “That’s what competition is all about.”

The director of special projects for the grocery workers’ union, UFCW 1500, Patrick Purcell, said he was disappointed by the mayor’s support for Costco. He noted that a report by the city Planning Commission earlier this year called for creating more supermarkets, a goal he said would be thwarted by competition from Costco.

“All this warehouse club is going to do is close markets,” Mr. Purcell said in an interview. “It goes counter to everything the administration is doing to bring supermarkets into the city.”

A spokesman for the Neighborhood Retail Alliance, Richard Lipsky, said the Costco store would run counter to another administration priority: reducing traffic.

“It is incongruous for the mayor, who supported congestion pricing, to support one of the most auto-dependent retailers in the country,” Mr. Lipsky said.