http://en.wikipedia.org/wiki/Freedom_of_speech_in_the_United_States#Core_political_speech

Core political speech

This is the most highly guarded form of speech because of its purely expressive nature and importance to a functional republic. Restrictions placed upon core political speech must weather strict scrutiny analysis or they will be struck down. The primary exception to this rule would be within the context of the electoral process, whereby the Supreme Court has ruled that suffrage or standing for political office as a candidate are not political speech and thus can be subjected to significant regulations; such restrictions have been upheld in Buckley v. Valeo.

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