"Because the maximum time limit for meeting a margin deficiency is shorter than in a standard margin account, there is increased risk that a client’s portfolio margin account will be liquidated involuntarily, possibly causing losses to the client."
TDA has a third type of margin account besides regular and portfolio:
https://tlc.thinkorswim.com/center/howToTos/thinkManual/Miscellaneous/RBC-Margin
Does anyone know the difference between this and portfolio?