NEW YORK - A year after Mayor Michael Bloomberg orchestrated a change to city law so he could run for a third term, voters decided Tuesday whether to keep the billionaire in office for another four years.

Bloomberg, the richest man in New York and founder of the financial information company Bloomberg LP, is on track to spend more than $100 million on the most expensive self-financed campaign in U.S. history. That's more than 10 times the amount of his challenger, Democratic city Comptroller William Thompson Jr.

Bloomberg is heavily favored to win, with recent polls showing him up to 18 percentage points ahead. But his actual margin of victory is predicted to be far smaller than the nearly 20-point blowout he pulled off in 2005.

That could weaken his power and make his third term more difficult at City Hall, where Democrats poised to sweep into citywide offices indicated they would not shy away from disagreeing with the mayor.

"It will be a very different experience than what he experienced the last eight years," said Democrat Bill de Blasio, who was favored to win the job of city ombudsman.

Public opinion surveys find the mayor with a much narrower lead over Thompson this year than the lead he held in 2005, when Bloomberg steamrolled Fernando Ferrer by almost 20 points. On Monday, A Quinnipiac University survey of 1,360 likely voters had Thompson 12 points behind Bloomberg.

Analysts say the smaller expected margin is partly due to voter resentment over the way Bloomberg hastily persuaded the City Council to change the term-limit law last year so that he could run again. But the anger is not widely predicted to be enough for a Thompson upset.

"The term limits thing did hurt, but the numbers still say 'Bloomberg easy,'" said Maurice Carroll, Quinnipiac's polling director.

The former Republican mayor, who does not belong to any party but still ran on the GOP and Independence Party lines, had long insisted he supported term limits before changing course last year.

New Yorkers who voted against Bloomberg on Tuesday overwhelmingly mentioned his changed position on term limits and exorbitant spending. He spent $85 million to win re-election in 2005.

"I didn't like the idea that King Mike thinks he can buy anything he wants, including my vote," said Democrat Kevin Anterline, a 56-year-old university employee who voted for Thompson.

Marjorie Shea, a retired high school teacher, said the spending was "overkill" but she voted for him anyway, saying his wealth and businessman's mind makes him an independent thinker.

"The team he has in place is doing very well. And he's not beholden to anyone," said Shea, a Democrat voting on Manhattan's Upper West side.

Helen Newman, 32, voted for a Green Party candidate, but said she wasn't bothered by the mayor's spending.

"I didn't really see anyone show up who had a chance against him," she said. "But then I guess no one showed up who had a chance against him because he has so much money."

Bloomberg would become the first three-term mayor in the city since Democrat Ed Koch, who has endorsed Bloomberg.