New York City Mayor Michael Bloomberg is throwing his financial support behind Maine independent Senate candidate Angus King, who is battling a stronger-than-expected challenge from a Republican contender in a race that could help determine control of the Senate.

Bloomberg is one of the major backers behind a $500,000 ad campaign that began running on the Maine airwaves on Friday, the group running the ads said. The ads extol King as an independent voice who could help break gridlock in Washington.