Una posibilidad.

¡No dejes de participar con tu voto!

Me parecen opciones adecuadas para el contexto dado.

I think the terms "ballot issue," "ballot initiative" "ballot measure" and "ballot question" all refer to the same the thing, i.e., referenda/plebiscites.

Ballot issue refers to a proposal submitted to the people at an election for their approval or rejection. It includes initiatives, referenda, proposed constitutional amendments, recall questions, school levy questions, bond issue questions, or a ballot question.
http://definitions.uslegal.com/b/ballot-issue/

plebiscite
a vote by which the people of an entire country or district express an opinion for or against a proposal
http://www.merriam-webster.com/dictionary/plebiscite

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Note added at 7 hrs (2013-03-21 00:50:55 GMT)
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oops: that's "...all refer to the same thing, i.e., referenda/plebiscites" :-)

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Note added at 1 day5 hrs (2013-03-21 22:30:49 GMT)
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referéndum.
(Del lat. referendum, gerundivo de referre).
1. m. Procedimiento jurídico por el que se someten al voto popular leyes o actos administrativos cuya ratificación por el pueblo se propone.
www.rae.es

plebiscito.
(Del lat. plebiscītum).
1. m. Resolución tomada por todo un pueblo a pluralidad de votos.
2. m. Consulta que los poderes públicos someten al voto popular directo para que apruebe o rechace una determinada propuesta sobre soberanía, ciudadanía, poderes excepcionales, etc.
www.rae.es