As the Senate Ethics Committee continues to investigate New Jersey Democrat Robert Menendez, the questions arising from his relationship with a top campaign contributor are much like those asked in past congressional probes.

Senator Robert Menendez (D-NJ) (Stacy Proebstle, Townsquare Media NJ)

From the so-called Keating Five to more recent investigations, members of Congress have run into trouble when they cross the sometimes blurry line between helping a supporter and engaging in a quid pro quo.

Richard Painter was an ethics lawyer under President George W. Bush and is now a law professor at the University of Minnesota. He says that most senators help supporters and, in his words, "rarely get nailed for it."

Painter says that Menendez has come under scrutiny because he didn't dot the i's and cross the t's.