And how large is the disenfranchised bloc at this point?
Not large enough to make a difference, if by "disenfranchised" you mean "doesn't vote for either major party."
Here's the thing: In order for a third-party candidate to make any difference at all beyond acting as a spoiler, that candidate needs to WIN. Not just have a good showing. Not just have a great showing where they come in just barely behind. In a FPTP system, there's only the winner... and everyone else.
Until we have a third-party candidate who can actually win, a vote for a third party is not quite the same as voting for the candidate you're most ideologically opposed to, but it's not very different, either.
And don't get me wrong, I fucking hate that that's the situation we're in. But it is. I wish I could argue against the mathematics of it, but they're unavoidable.