Advocate versus Partisan
There’s no such thing as perfect synonymy. Like most of what we call ‘synonyms’, these two words have a broad area of meaning in common, but vary at their edges, like two circles which overlap but do not coincide.
partisan means a zealous supporter of a faction or party. From the beginning, partisan had negative connotations: it implied blind and unthinking adherence to one’s faction’s positions. Moreover, when the word first came into English, factions were not the kinder, gentler political parties they are today; they were often violent and occasionally insurrectionary gangs; and partisan today still carries a suggestion of at least rhetorical violence.
advocate means a zealous supporter of a cause … Note that there’s nothing in that that’s really different from ‘zealous supporter of a faction’; but it sounds different. When advocate first came into English it meant a ‘lawyer’, somebody who argued your cause before a court of justice; and a lawyer as we all know is a gentleman of great learning and professional probity, quite different from a street thug. To this day, advocate is used primarily for one who is an eloquent spokesman for a cause rather than a fighter for a cause.
What it boils down to (ignoring the areas where these words have developed quite different meanings) is that My cause is represented by a rational, principled, passionately engaged advocate, while Your faction relies on vicious and ignorant partisans.
partisan : n
1. An adherent to a party or faction.
2. A fervent, sometimes militant, supporter or proponent of a party, cause, faction, person, or idea.
3. A guerilla fighter; a member of detached light troops acting behind enemy lines.
4. (now rare) The commander of a body of detached light troops engaged in making forays and harassing an enemy.advocate : n
1. Someone whose job is to speak for someone's case in a court of law; a counsel. [from 14th c.]
2. Anyone who argues the case of another; an intercessor. [from 14th c.]
3. A person who speaks in support of something. [from 18th c.]
4. A person who supports others to make their voices heard, or ideally for them to speak up for themselves.adherent : n A person who has membership in some group, association or religion.
From the above an advocate is someone who speaks or argues for a cause that may not be their own while a partisan belongs to the group, faction, party, for which they are arguing.
The phrase partisan advocate could be describing someone who is an advocate who is arguing along party lines or it could be talking about somebody who advocates on behalf of the partisans. Context should tell you which one is applicable in your case.
You will find references to a partisan advocate in order to distinguish that role from that of a dispassionate advocate.
When an (alleged) mass murderer and ethnic cleanser is brought before the International Criminal Court, he is entitled to have his defence presented by someone as skilled as the prosecutor. We trust that the person who does this is not also an adherent to the distasteful beliefs underling the crimes.
Last year the revolutionary government of Libya asserted that the advocate for Saif al Islam (son of Gaddafi) was exceeding that role and acting in a partisan manner. Her rebuttal of that claim rested upon the difference between the two words.
It might be supposed (in the context of your speech class) the most effective persuasive argument would come from a skilled advocate who has a partisan commitment to the case.