"参政权扩大论者" can be translated as "advocates for the expansion of political participation rights" or "proponents of broadening suffrage/voting rights" (depending on the specific context of "参政权," which generally refers to political participation rights, including but not limited to voting rights).
If emphasizing the broader concept of political participation beyond just voting, "advocates for the expansion of political participation rights" would be more appropriate.
If specifically referring to the expansion of voting rights or suffrage, "proponents of broadening suffrage/voting rights" could be used.
In most general contexts, "advocates for the expansion of political participation rights" is a suitable and comprehensive translation.