I just had a fairly terrifying thought.
If a strong, viable and popular third party party can't rise up in the middle of THIS partisan disaster, maybe it never CAN.
Is it possible that independent and moderate voices are now so unwelcome in both "major" parties that they will no longer be allowed to see the light of day? Did we, as country, shut the door to independent and moderate thought so quickly and so firmly that they simply no longer have a place?
I hope I'm wrong here.
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A third party would need to incorporate a philosophical, cultural tradition to which a huge chunk of the electorate could relate. What tradition - outside the (economic) libertarian-conservative / liberal-progressive traditions - could that be? The OP doesn't say, and probably doesn't know.
A third party would need a galvanizing set of issues, societal / political problems afflicting a huge chunk of the electorate, which this new party, as empowered by its philosophical outlook, would be uniquely positioned to solve, whereas the other major parties are not. What issues could this be? The OP doesn't say, and probably doesn't know.
The current front-runner in the Democratic field is, last I checked, Biden - a solid centrist. Buttigieg is a centrist. Harris is arguably a centrist. Klobuchar is a centrist. That's just those off the top of my head; I am sure there are more. The OP's fearful contention that "independent and moderate voices [are] now [...] unwelcome in both 'major' parties" is quite obviously otherworldly. Not to mention incorrect. The underlying contention is the figment of the OP's very active imagination, the "hard-core left" that has taken over the Democratic party, when there is no such thing as a hard-core left in the U.S., none of any import or influence.
There is, however, a hard-core right, and, arguably, it has taken over the GOP. So, if there is a third party to be formed, it needs to rescue whatever serious, reasonable conservative thought and policy-making can be rescued from the current-day reactionary train-wreck that is the GOP, and hope to get national notoriety fast, and not least quite a number donors lined up.
All that would, of course, collapse into a two-party-system again, since the winner-takes-all voting system pretty much preforms that. The third party that poses the highest risk of a "vote thrown away" in a FPTP system dies in short order. How to change that voting system, and with what to replace it... the OP doesn't say, and probably doesn't know. He's fine grousing about the hard-core left he invented. The two parties remaining would, of course, be subject to the exact same (at least pretty much the same) societal, financial, economical forces as are the parties right now, they would have to fight for their respective bases' attention and support just as they do right now, and in short order the creation of that fabulous third party would result in the exact same thing the OP bemoans and decries, and has been for years. The lesson is, a third party doesn't solve a thing, particularly not the problems of an apathetic, ignorant electorate with a gigantic surplus of resentment and the attention-span of a gnat.
So, finally, how is it ever too late for anything? How is it possible to state with any confidence the conditions for an event are right at this time, and then never again? Short of nuclear self-elimination, that never happens, in the exact same way as "the end of history" never happens. Never. It was, however, "a fairly terrifying thought".