With election season officially under way, unions are busy examining whether Mayor Bloomberg or the Democratic nominee, Fernando Ferrer, would give them the best deal.

With a number of major union endorsements still up in the air, it’s unclear who is handing out the most desirable promises to unions, but what does seem clear is that in the world of political endorsements, deal-making is the norm rather than the exception. Under one proposed deal, the mayor would have received the health-care workers union’s endorsement in exchange for hiring 25,000 new home health-care aides. Mr. Bloomberg last week rejected the proposal.

“Interest groups are looking for favorable treatment from the government, and when public officials provide that treatment, they expect that the interest groups will either not oppose them or will support them,” a political consultant, Norman Adler, said. Mr. Adler, who was the director of political action and legislation for District Council 37 for 11 years, added that while deal-making is a normal part of the endorsement process, the deals are rarely as clear-cut as the one Mr. Bloomberg turned down last week.

Mr. Adler cautioned: “You’ve got to be careful about these things. I don’t think anybody ever says to anybody, ‘You do this and you get that.'”

A DC-37 official, who asked not to be identified, said all unions give support “based on how you think the candidate is going to help your members.” However, the official said support is not usually traded explicitly for favors, saying, “People don’t even talk to one another like that.”

So while most unions probably don’t offer endorsements in exchange for specific favors, many withhold endorsements from politicians who haven’t met their demands.

Yesterday, a day before Mr. Ferrer is expected to receive the coveted health-care workers endorsement, Mr. Bloomberg tried to distance himself from endorsement negotiations, but he stood behind his administration’s contention that there’s no money to boost the size of the city’s workforce by 8%.

“I’m not going to comment on any conversations that I or the campaign have had,” he said. “But the fact of the matter is, this city, with a deficit staring us in the face and our existing 300,000 employees wanting to get more so that they can share in the great American dream, we just cannot go ahead and add 25,000 people to our workforce. It could cost up to $1 billion by some estimates and that’s just not an idea that I could support.”

Later, Mr. Bloomberg’s senior campaign adviser, William Cunningham, called the health-care workers proposal “breathtaking.”

“I don’t talk to the unions, but I’ve never heard of anything like this,” he said, adding that the other union endorsements the mayor has received have been based on his record, not on backroom arrangements.

Mr. Ferrer was less direct about his dealings with Dennis Rivera’s healthcare workers union, 1199 SEIU, which is announcing its support for him today.

At first, he didn’t provide answer whether he would bargain with taxpayer money to receive the union’s endorsement. When pressed, he said, “We are concerned about the same kinds of people. We have [been] throughout his career as president at 1199 and throughout my public career.”

A Ferrer spokeswoman, Christy Setzer, said her candidate had “absolutely not” signed onto any deal with 1199 in exchange for an endorsement.

1199 officials did not return multiple requests for comment.