What is the main issue of campaigns for third party candidates in US elections?

In the middle of bipartisan domination, the votes obtained by third party candidates can unbalance the general one. The US electoral system is designed by and for bipartisanship. Even the third party percentage of voters is generally lower than republicans and democrats, it has been used as an opinion stroke in order to decide the presidential fate in several states of US.

Third party candidates can be spoilsport for Democratic or Republican candidates.

Some Bush campaign officials believed that Perot spoiled Bush's reelection.