Losing. Look at the 1992 presidential election, about the high water mark for third party national voting. Yet the third party vote was either ineffective, or actively harmful to the electoral chances of Bush. The next election third party voting fell off a cliff.
The reason it is unstable is because with single non transferable votes you very easily have situations where theoeast favored candidate wins, due to the two more favored splitting votes too much.
Let’s say candidate A is the left, B the centrist, and C the right
A is the favorite of 35%, and the second choice of 25%
B is the favorite of 25% and second choice of 50%
C is the favorite of 40%, and second choice of 0%
10% of A have no second choice, and 15% of C have no second choice
In this scenario the centrist is the most popular, or at least most acceptable to the broadest swath of people. A decent compromise candidate. But there is a portion of hardliners on both sides who would be unhappy with them, 25%.
However, because voting is winner take all here, then the person with the least broad appeal wins.
And people notice. Note how many people were (wrongly I’d add) blaming Bernie voters, or Jill Stein and Gary Johnson voters, for Clinton losing. How many people got mad at third party voters, and blamed them for Trump? How many people who voted for them, say a Stein voter in Wisconsin, regretted what they thought was a harmless protest vote because Trump won? That, had they known, they would have voted Clinton?
Do you think those people would be more, or less, likely to vote for a third party? They’d go back to strategic voting. Not voting for who they really want perhaps, particularly in primaries even, but voting for who they see had the best chance of beating the person they really despise.
No, a third party would gain some support perhaps, and in best case scenario may even win, in which case they may totally supplant one of the existing parties. At which point the Whig party collapses and ceases to be within a decade, replaced by the Republican Party.
No electorally third parties are not stable members of our government. They either get coopted by one of the two major parties, or they actually cost elections for one of them, and garner a backlash as people are less happy about their least favored party winning instead.