Answer: when it causes you to screw over your party:
Yesterday the Virginia GOP came very close to taking control of the state Senate, nearly luring a Democratic Senator to switch parties and put them at a 20-20 tie, which would have been broken by the Republican Lt. Governor. Then Jeff Frederick, a state legislator and the party chairman, ruined it all by Twittering this:
Big news coming out of Senate: Apparently one dem is either switching or leaving the dem caucus. Negotiations for power sharing underway.
The Dems then read the message, quickly mobilized to talk the renegade out of it, and stopped the GOP coup before it could happen.