Third parties generally serve only to take votes from one of the main parties. Third parties generally serve only to take votes from one of the main parties. If the third party has a candidate promising similar things to the Rep. candidate, the Democrats are more likely to win simply because their votes are not being siphoned off. Think of it this way: if a school class were to vote on favorite colors and there were only blue and red to choose from, it would probably be mostly equal. But if the same class were to vote for blue, teal, and red, I would be willing to bet that red would win just because its votes weren't being split like the blue votes were. Teal is the third party. It won't ever win on its own, but it can be influential in that it can help the opposite party win.