Some people, mostly on the left, have blamed Trump’s victory on third party voters. Here, I am going to look at how the election would have likely gone if there were no third party voters using CNN exit polls which asked voters who they would have voted for in a two way race between Trump and Clinton.
In terms of the national popular vote, a two-way race would have produced a margin of victory of less than 1%. Since this is what actually happened in the election, this suggests that third party candidates did not systematically change the vote in favor of Clinton or Trump.
In terms of important swing states, Trump would have still won Arizona, Florida, North Carolina, and Ohio. He would have lost Michigan. In both Wisconsin and Pennsylvania, a two-person race would have produced a margin of victory of less than 1% and the CNN exit poll service does not let us see who the winner would have been in this close of a race. (This isn’t very different from what actually happened in PA, where Trump won by a single point.)
For Clinton to have won, she would have needed to win both states. If one went to her and one went to Trump, Trump still would have won.
The existence of third party candidates did seem to help Trump in key swing states. That being said, even in a two-party race Trump still would have had an advantage in electoral votes excluding WI and PA, and we don’t have good reason to think that he would not have won at least one of those states.
That is assuming that all the third party voters still showed up. This seems unlikely. Some of them, probably more than the number who will admit it to pollsters, would not have voted at all in a two-way race. If this is true to any significant degree, Trump would have probably still won both PA and WI.
This also assumes that third party voters are being honest about who they would have voted for in a two party race. Given that it is widely believed that Trump voters lied to pollsters throughout this election by pretending to not support Trump when they actually did (because supporting Trump was stigmatized) this also seems unlikely. Once again, if this is true to any significant degree, Trump would have probably still won both PA and WI.
In light of these two considerations, I think it is pretty likely that Trump would have won a two-way race. Even if not, it makes little sense to “blame” 3rd party voters. They were simply voting for the candidate who most matched their views.
Moreover, even if their candidate did not win, supporting a 3rd party candidate can help grow the party itself and strengthen it for future elections. This is especially true if a candidate is polling near 5%. If a 3rd party reaches that threshold, they get federal funding for future elections. This consideration was likely very relevant for Johnson voters since Johnson was polling at over 5% for much of the election and was averaging at 4.7% in polls taken right before the election. (He ultimately got 3.3% of the vote.)
Voting third party can also make sense from the perspective of getting major parties to listen to you. If, for instance, the democrat party thinks that it was significantly hurt by Stein voters then it will probably try to cater to those voters in the future.
Thus, voting third party is not only justifiable in terms of ideology, but it can also be practical.
It is worth noting that both the libertarian party and the green party chose absolutely horrible candidates this year. According to data assembled by HuffPollster, both of them had net unfavorability ratings on par with Trump and Clinton, who were notoriously unliked:
These third party candidates did not attract millions of new voters because they were great politicians. If the democrats want fewer people to vote for third parties, they should run better candidates. The fact that all four parties, the two major ones and the two largest 3rd parties, all ran extremely unliked candidates this year suggests that, if anything, both the major parties got lucky. Far more votes could have easily gone to the libertarian and green party had they run better candidates. Both parties should work on bringing 3rd party voters back to them. Attacking them for their vote is not the way to do it.