Learn English – ny difference between ‘often’ and ‘frequently’

"Voters" are the people who vote (or more generally, those who are entitled to vote, whether they do so or not).

"Electorate" is usually a mass noun meaning "the collection of all voters". The use of the plural in your quotation I find rather strange and can only interpret it as "the collections of voters in different countries or polities" - its plural form occurring only because this is a general text talking about different countries.

"Constituents" are the people represented by a politician. I would use it to mean all those represented, whether they are voters or not, but there is room for some argument there. In most places "constituents" are a geographically defined group, but they could be, for example, the members of a profession if some body were organised to have representatives of professions.

Surely there should be more of a difference between the two words. If they mean the same thing surely there would be no need anymore for two words

There are a lot of near synonyms in English, so this does not follow at all. It's particularly so when you consider that act entered the English language in the late Middle English period, from French, while deed was in the language from the very beginnings of Old English. Many, but not all, seeming redundancies in English vocabulary have a similar origin story.

It is true that there are differences in nuances. The primary one would be that an act would generally be need to be relatively remarkable before we would use the term deed. One can do a particularly brave or noble or dastardly or evil deed, but one can't really do a lazy or everyday or common deed. There's nothing technically wrong with the latter, but it wouldn't be an idiomatic use.

There are similarly some differences in what verbs and prepositions are used with either. One doesn't do an act, one cannot be in the deed of anything.