To make this argument, we need to look at the potential impacts of the two kinds of stereotyping. When individuals stereotype us, it can be personally hurtful. If the individuals have power over us (say as employers or teachers) they can do us tangible harm. But when a government or political party engages in stereotyping, the harm is much worse. The government can create policies that affect millions of people and are based on stereotypes. Political parties can help to make millions of people believe in the stereotypes as they present those stereotypes in the speeches of their candidates.
In these ways, stereotyping is much worse when done by governments or political parties because it has an impact on a much larger number of people.