The polls in New York City are open and while Millionaire Mike is the odds on favorite, Thompson could score an upset.
Voting started at 6 a.m. and polling centers are open until 9 p.m. in New York.
Both mayoral candidates voted early, with Thompson casting his ballot in Harlem and Bloomberg buying some bake sale cookies after voting at an Upper East Side school.
"Does that include sales tax?" the mayor asked while forking over the $1 for the treats.
Speaking of money, Bloomberg's record spending is favored to win him a third term on Tuesday, but by a far smaller margin than the near-20-point blowout he pulled off in 2005.
Public opinion surveys find Bloomberg with a much narrower lead over Democrat William Thompson Jr. this year than the lead he held over his Democratic opponent in his last re-election bid, when he steamrolled Fernando Ferrer by nearly 20 points.
The billionaire mayor spent record sums from his personal fortune on both elections. He's likely to burn through more than $100 million on this one, while Thompson is expected to spend one-tenth of that. The mayor spent $85 million in 2005.
Analysts say the smaller margin expected this year is partly due to voter resentment over the way the mayor hastily persuaded the City Council to change term-limit law last year so that he could run again.
The anger has simmered, but is not widely predicted to boil up enough for a Thompson upset.
A Bloomberg win "will be less than the landslide it was four years ago," said Maurice Carroll, Quinnipiac University polling director. "The term limits thing did hurt — but the numbers still say 'Bloomberg easy,'" he said.
The latest poll on Monday found Thompson 12 points behind Bloomberg, tighter than the 18-point gap in a Quinnipiac poll a week ago. A Marist College poll last Thursday had Bloomberg up by 15 points.
Monday's Quinnipiac poll said 50 percent of the 1,360 likely voters surveyed support Bloomberg, while 38 percent backed Thompson, the city's comptroller. About 10 percent were undecided.
The former Republican mayor, now not in any party but still running on the GOP and Independence Party lines, had long insisted he supported term limits before changing course last year.
The latest Quinnipiac poll had a plus or minus 2.7 percentage point margin of error.