PHILADELPHIA — Mayor Bloomberg hit the campaign trail yesterday, endorsing Pennsylvania Senate candidate Joe Sestak as he hailed the Democrat’s political “independence” and position on fighting illegal guns — an issue Bloomberg has made a priority.

Sestak, a congressman who toppled incumbent Sen. Arlen Specter this year in a primary, is co-sponsoring a bill to tighten regulations around gun shows, which the mayor noted in his speech.

“He’s protecting law-enforcement officers everywhere by supporting policies that will keep guns out of the hands of criminals,” said Bloomberg, who has used his vast wealth to privately promote this issue.

Sestak is trailing GOP former Rep. Pat Toomey.