Third party votes have frequently changed
politics. A few third party candidates have been elected to
Congress, but never has a third party candidate been elected
President. What usually happens is the two major parties absorb the third party's
platform.
During the 1800's, farmers were having many problems. They would borrow money to build a farm, but then they would have problems repaying the debt. The supply of hard currency was scarce, and farmers had trouble getting a hold of gold and silver to pay back their loans. This resulted in the formation of many political groups, namely the Grange and the Greenback Labor party. Candidates ran on the Greenback Labor party ticket. Several Congressmen were elected. The Republican and Democrat parties noticed the support that this upstart group had gained, so they fought for the votes by including the Greenback Labor party's wants into their own platforms and campaigned on them. This sucked the support out of the Greenback Labor party.
The final result was the third party's needs were met. Farmers voted for a third party and things were changed in their favor.