The description of Silver Badges on the Badge Page says:
Silver badges are awarded for longer term goals. Silver badges are uncommon, but definitely attainable if you’re interested.
The silver Constituent badge, however, is awarded for voting in a moderator election. To earn it, you only have to click one button. There are bronze badges that take more work to earn - like Quorum, Student or Teacher, which at least require that you get some upvotes.
I argue that voting in a moderator election is basic use of an SE site, and the badges you get for this should be bronze, just like the Caucus badge:
Bronze badges are awarded for basic use of Stack Overflow. They are easy to earn.
I could also put this another way. Suppose someone is willing to vote in a moderator election if it earns him/her a silver badge, but will no do so if it only earns him/her a bronze badge. I argue that this hypothetical person is hardly taking moderator elections serious anyway.
So, a bronze badge is high enough for voting in a moderator election.