One way to tell when they cross the line is to watch the comics and jokes circuit. Here's another one:
Original source is RollCall.com, so tap their phones, not mine.
Seriously though, the reason this scandal has "legs" is because the Democrats' hands are clean. In previous scandals, it seems that the dirt was evenly spread across the political dipole. This time, the Dems have one they can get their teeth into and not look silly.