Many states have a much closer margin, so swaying a few % can make or break a campaign. See the California case and all the money the Mormons dumped into the P

Many states have a much closer margin, so swaying a few % can make or break a campaign. See the California case and all the money the Mormons dumped into the Prop 8 campaign. They used ads that basically lied about what allowing gay marriage would mean (saying things like schools would start encouraging kids to be gay), which helped scare just enough people.