Although the American system is dominated by two main parties (Republican and Democrat) there are numerous smaller parties that support particular issues and even run candidates.
Source
Reform party http://www.reformparty.org/; RNC logo. Created by Gary Keith and Mike Frost. http://en.wikipedia.org/wiki/Image:Republicanlogo.png; DNC logo http://en.wikipedia.org/wiki/Image:Democratslogo.png; America First logo http://www.americafirstparty.org/; American Party http://www.theamericanparty.org/; Green http://en.wikipedia.org/wiki/United_States_Green_Party; Libertarian http://www.lp.org/; Socialist http://www.sp-usa.org/