According to the US Census, you are not Hispanic. We can argue about semantics, but that's kind of like the white South African arguing that they are African-American. We all know what Hispanic is SUPPOSED to stand for, and regardless of whether you're 50% Portuguese or 100% Portuguese, you do not fall into that category.
EDIT: Here's a direct quote from the census. It actually makes it seem like Spain could be interpreted to be part of this group, but I really do think that it is understood that Spaniards are not included when we are talking about the Hispanic experience or challenges Hispanics may face in our society. Other sites specifically note that Portuguese speakers are NOT considered Hispanic by the census.
Hispanic Origin. Persons of Hispanic origin were identified by a question that asked for self-identification of the person's origin or descent. Respondents were asked to select their origin (and the origin of other household members) from a "flash card" listing ethnic origins. Persons of Hispanic origin, in particular, were those who indicated that their origin was Mexican, Puerto Rican, Cuban, Central or South American, or some other Hispanic origin. It should be noted that persons of Hispanic origin may be of any race.
And the source:
http://www.census.gov/population/www/socdemo/hispanic/hispdef.html