"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."
http://www.scotusblog.com/case-files/cases/citizens-united-v-federal-election-commission/
So if bitcoin is considered a currency (in additional to being a protocol), it should be legal to spend it as a form of protected political speech.
You are correct. I work at a political action committee (IRS 527) that focuses specifically on accepting bitcoin as contributions.
www.coinpac.org