Cartão de Cidadão (former Bilhete de Identidade) is a multipurpose document. Besides voting it is also accepted for travel in the EU etc. BC
It is my personal opinion that mere re-registration in Lisbon of births taken place in Goa prior to 19th December, 1961 does not necessarily mean that one becomes a Portuguese citizen. One has to apply further for a Cartao de Cidadado (something alike our Identity Card for voting purposes) and based on this Cartao one could apply for a Portuguese passport if one wishes.
