New York City’s economy may lose as much as $281 million during the four days it plays host to the Republican National Convention at the end of the month, according to an analysis by the city comptroller’s office obtained by The New York Sun.

Added security costs borne directly by the city and not reimbursed by the federal government will subtract another $28 million, according to the comptroller’s office.

Mayor Bloomberg has been saying for months that the convention would be a boon, not a bust, for the city. Mr. Bloomberg managed to raise about $70 million in private funds for convention-related expenses, including renting Madison Square Garden and constructing a temporary walkway that will connect the Garden with the post office across the street. The convention will make the otherwise dog days of summer a boon for the city, he said, as visiting delegates and reporters patronize the city’s restaurants and hotels.

“No one expects it to look like the week before Christmas,” Mr. Bloomberg told reporters at CUNY yesterday. “But we do hope the extra business makes up for the normal shortfall of activity” that occurs in late August when the city clears out and tourists opt for cooler destinations.

Some analysts said that the mayor may be overly optimistic. The Connecticut Center for Economic Analysis at the University of Connecticut has found that city leaders often look at convention dollars through decidedly rose-colored lenses. It focuses on the economic activity but don’t consider what the city might have taken in anyway. It also often neglects to calculate productivity losses and other issues that take a bite out of regular economic activity.

According to NYC & Company, which is in charge of drumming up tourism for the city, the convention will bring about 50,000 people – including 15,000 members of the press – to New York and will generate more than $265 million in economic activity. It also expects 1,800 temporary jobs and $10 million in tax revenues.

The comptroller’s office adds the figures differently. It focuses on the gross city product, or GCP, which measures goods and services produced by people who work in the city. On average, the city’s GCP on a weekday is about $1.5 billion and about $500 million on a weekend day, the figures said. Manhattan accounts for about three-quarters of that, according to the comptroller’s estimates. Comptroller analysts then predicted that lost productivity due to commuter delays and a business slowdown would reduce activity in Manhattan by one-sixteenth during the convention. That works out to $70.3 million a day, or a potential loss of $281.2 million for the entire convention.

Officials at the host committee were apoplectic that six days before the convention the comptroller, William Thompson Jr., a Democrat, would doom cast about the event’s prospects. “Here we have the city comptroller scurrying about in dark of night trying to get headlines off the back of thousands of New Yorkers employed by this convention,” said host committee press

secretary, Paul Elliott. The city stood by its contention that the convention would provide a boost to an otherwise slow August, he said.

To be sure, the convention has boosted spending. City officials have said security for the RNC could end up costing as much as $78 million, most of which will be reimbursed. The federal government has kicked in $25 million and the Senate has approved an additional $25 million to defray those costs.

That money will likely be gobbled up in overtime costs for the police alone. There will be 10,000 officers around Madison Square Garden and other officers will be called in to cover their regular beats.

Another $12 million is being spent on training in counterterrorism and First Amendment rights, so officers can deal with protesters without violating their civil rights. The city spent about $17.5 million on equipment such as Delta barriers, which are the metal lift plates that can stop a speeding truck.

Analysts expect the city government will end up picking up about $28 million in security costs. Add that to the $281 million in possible business losses the comptroller forecasts and the city could end up $309 million in the red, the analysis said. The comptroller looked at the numbers, aides said, after reporters’ queries about the cost of the convention.

The Beacon Hill Institute at Suffolk University originally forecast the July Democratic convention would cost Boston some $12.8 million. But a post convention analysis released by Beacon Hill last week found that with net new spending and adjusting for multiplier effects the city likely ended up $14.8 million in the black.

The City of Boston had estimated that the convention would add about $154 million in economic benefits to the city. The actually number was closer to $156.7 million, according to Beacon Hill. But the Beacon Hill Institute said the convention sucked about $141.9 million out of the economy in lost business because it displaced events and Boston lost tourist and commuter spending.

Speaking to reporters yesterday, Mr. Bloomberg was eager to draw a distinction between the way New York was handling the convention and the way Boston handled its convention.

Boston officials had also suggested residents take a vacation and get out of the city during the political fest. Mr. Bloomberg has decidedly gone the other way.

“You should go about your business,” he said yesterday. There would be inconveniences for New Yorkers, he said, but they would be minimal.