It's a failure on multiple levels. We'd do history a disservice if we don't examine them all and look for ways to fix them.

Voter disillusionment is a thing, and the Democrats failed to properly engage enough voters. The worsening economic situation the Democrats failed to do enough about created an environment known to foster nationalist and supremacist ideologies.

Disinformation campaigns make it worse. All the "vote third party" spoiler advocates and "don't vote for genocide" anti-electionists didn't help. The electorate system making third parties spoilers in the first place hamstrung actual democratic representation. The corporate controlled media bias and refusal to call the fash out for what they are legitimised them.

Free Speech apologists defended the right to spew anti-democratic rhetorics. The "high road" numbskulls left the field to the dirty, but effective tactics of those willing to sacrifice all decency for power. A failure to clearly and understandably communicate the intention of complex proposals favoured the simple, emotionally appealing talking points of the right.

Yes, the voters bear blame too: Republican voters for actively enabling fascism, complacent non-/third-party-voters for failing to effectively and strategically oppose them (moral opposition is good and right, but it doesn't win wars - strategy and cohesion do). But only attacking the symptoms of a fucked up accumulation of problems doesn't solve the root causes.