The easiest answer would be the 1st Amendment to the U.S. Constitution. "Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the government for a redress of grievances."
By ensuring that its citizens had the right to practice any faith, speak their mind, write in print, and assemble for meetings, they protect their citizens ability to be involved in politics.
