Why do Democrats fear a 3rd Party Candidate?

[SunsetBengal]

If what Pollster Mark Penn is reporting is true, that 64% of all voters want another option to vote for, one would think that the time would be right for a 3rd party candidate. I'm not so sure that Trump would garner anything close to 70-75% of conservative voters' support if there were any sort of viable non-democrat party alternative.