The definition of protagonist most familiar to me as a student of literature is this one:
protagonist noun: the chief character in a dramatic work. Hence, in extended use: the leading character, or one of the main characters, in any narrative work, as a poem, novel, film, etc.
According to this definition, the sister of Polynices is the protagonist of Antigone, Dorothea Brooke is the protagonist of Middlemarch, and Ree Dolly is the protagonist of Winter’s Bone.
The use of protagonist in an article about a prison-related work program alerted me to another use of the word:
This program, termed “evil” by protagonists because the prisoners aren’t paid prevailing wages, has changed the lives of prisoners.
In this context, protagonists refers to people who object to the work program and advocate its abolition.
Here are other examples of protagonist used in nonliterary contexts:
Two activists, Shirley Andrews and Barry Christophers, became the chief protagonists in the Federal Council’s fight for equal wages.
Accordingly, management and male workers were the protagonists in these struggles.
The leading protagonist of “women’s rights” was a remarkable woman of frontier type named Abigail Scott Duniway.
[HK169, (a trade union)] has been the most prominent protagonist in equal value cases and took the Danfoss litigation
The U.S. must be careful “not to get engaged in such a way that we become the chief protagonist, and eventually not just in Syria, but in the region as a whole,” Mr. Brzenzinski said.
In addition to the literary definition of protagonist, the OED offers these:
2 a. The main figure, or one of the most prominent figures, in any situation; a prominent supporter or champion of a cause.
2 b. In weakened use (without connotations of prominence): a proponent, advocate, or defender of a cause, idea, etc.
3 a. leading player or competitor in a game or sport, or on a team.
The first two citations given for 2a suggest that this use of protagonist began as a figurative application of the meaning “chief character in a dramatic work”:
Those republican demagogues [in France] who acted the part of iniquity became themselves the protagonists of the piece and perished in the catastrophe. (1801)
If he [Lafayette] has not been the Alpha and the Omega of the era…,if he has not always been the protagonist of its different acts, yet has the whole of his somewhat extended life been devoted to the cause. (1837)
The weakened use described in 2b may have arisen from the mistaken idea that the pro- in protagonist means the same as the pro- in a word like pro-government.
In fact, the first three letters in protagonist do not bear the meaning “for” (as in “for and against”). This pro derives from Greek proto, “first.” The protagonist is the first (or chief) actor.
When referring to people who play an important part in promoting a cause, the word protagonist is appropriate. Using it as just another word for the noun advocate is less so.
Here are other words that mean supporter or advocate: