New York City's independent mayor is weighing in on Pennsylvania's Senate race, endorsing Rep. Joe Sestak (D) over former Rep. Pat Toomey (R).  

Mayor Michael Bloomberg will appear with Sestak at an event in Philadelphia on Tuesday, but the timing is also likely to inject the controversy over the Ground Zero mosque into the event.

The mayor has been one of the project's most forceful defenders, while Sestak declined to endorse the project Monday.

A spokesman for Sestak said the congressman "believes there is a Constitutional right to religious freedom and separation of church and state that applies equally to all Americans," but he declined to clearly back the plan.  

Spokesman Jonathon Dworkin said Sestak "is not looking to say what’s best for New York as long as that right is respected — he is focused on Pennsylvania."

Bloomberg hasn't exactly been a hot commodity on the midterm campaign trail, but he did host a fundraiser for the Senate campaign of Rep. Mark Kirk (R-Ill.) last month.

The Democrat-turned-Republican-turned-independent Bloomberg can also be a political lightening rod at times. 

While Sestak has taken some lumps in central and western Pennsylvania over his support of gun control, Bloomberg is one of the country's most ardent proponents of stricter gun-control laws. He founded the group "Mayors against illegal guns," which spent millions in independent expenditure dollars against anti-gun-control congressional candidates during the 2008 cycle.