Is online voting open to some degree of coercion? Yes, indeed it is, in the exact same way that absentee ballots are.
Conceivably a fraternity or organization could coerce its members to get absentee ballots and hand them over unmarked in an effort to steal the election. Well, guess what? We allow absentee ballots and that scenario doesn't happen.
Want my guess as to why? Because doing so is illegal, and ultimately, this is just SG we are talking about here.
Online voting may or may not facilitate the process, but the cost-benefit analysis is still the same, and it's my hope that students are smart enough to recognize Student Government is not worth felony voter fraud charges. So, my suggestion to SG politicos: Let online voting onto the ballot. Students obviously want it, and other schools demonstrate the workability of the system.
What more is there to say?
