The mandatory bar is only displayed for 48 hours - this one cannot be dismissed.
However, one notice was inserted into everyone's notification bar (the one that tells you if you've gained a new badge, etc) so that if one did not log in during the mandatory bar period, one still gets notice about the election.
The reason the mandatory bar is only on for 48 hours is that people complain about not being able to get rid of it.
The reason to have a mandatory bar for at least 48 hours is to make certain those most active users are very strongly encouraged to participate, whereas it's easy to get rid of the notice bar message thinking one might get to it later, but then forgetting to do so. The mandatory bar is somewhat in-your-face.
Personally I'd prefer a slightly different mix: The mandatory bar appears on all accounts, is not dismissable, and does so for the whole period of the election, but only for those users who have not completed their vote who are eligible to vote.