3rd parties are viewed as potential "spoilers" in elections. in 1992 Ross Perot who failed to get the Republican nomination ran as an Independent and may have captured enough conservative votes to allow Bill Clinton to win the presidential election. The 2000 election Bush won Florida that ultimately decided the national election by a mere 527 votes. No doubt more of Ralph Nadar's 97,000 votes would have gone to Gore than to Bush so had Nader not run Al Gore would almost certainly have been the 43rd president of the United States. Of course if Democratic leaning precincts didn't run out of ballots, 2 counties hadn't lost the votes due to computer crashes, if thousands of voters weren't wrongly disenfranchised as felons who weren't, and if many Gore voters hadn't mistakenly chosen a different candidate due to a confusing butterfly ballot Gore would have been president anyway.
I agree with WestCoastBro they do view them as irrelevant except that they could harness some of the voters enough to make a difference for one or the other.
I think Democrats & Republicans alike view third parties was irrelevant.
*as (why is there no edit button?!?!)