This is what the Supreme Court said in the Citizens United case:
Holding: Political spending is a form of protected speech under the First Amendment, and the government may not keep corporations or unions from spending money to support or denounce individual candidates in elections. While corporations or unions may not give money directly to campaigns, they may seek to persuade the voting public through other means, including ads, especially where these ads were not broadcast. From the SCOTUS WEBSITE
The real issue is are you more influenced by an ad financed by the SEIU, a conservstive PAC financed by the Koch brothers or an ad run by the Sanders campaign? No doubt each ad is likely one-sided, perhaps is misleading and devoid of substance.