swing factor

An external force or forces (eg. person, event, place, thing, etc.) that contribute(s) to making a decision either one way or the other.

Tom: I'm not sure if I'm going to Derek's party tonight.

Dave: Tracey will be there. I heard through the grape vine that she gets down with one dude and another girl on the first Saturday of every month. Guess what tonight is!

Tom: Well there's my swing factor.