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Reasons to vote for a 3rd party candidate
1) The person represents your views on society.
2) Show your displeasure with the two-party system. This is a powerful message to political officials. If thousands of individuals who feel left out of both the Democratic and Republican party vote for a third party candidate, it will shift the way politicians approach third-party candidates.
Reason why it would be considered a "waste"
If a third-party candidate has no chance of winning, some would consider it a waste. This is due to the fact that your vote could have been used to support a candidate with somewhat similar views who actually had a chance at winning.