The advertisement, which was scheduled to start running on Saturday, shows Mr. Obama praising Mr. Brown for sponsoring a bill to end insider trading in Congress. As Mr. Obama signs the measure into law, he thanks Mr. Brown, saying, “Good job.”…

That approach would be unusual enough for a Democrat in a year in which many feel the need to distance themselves from the administration, but it is virtually unfathomable for a Republican. That both candidates are trying to leverage their ties to Mr. Obama underscores how popular the president is here and how unpopular his rival, Mitt Romney, is, even though he once served as the state’s governor. Mr. Obama is expected to carry Massachusetts overwhelmingly in November.