The democrats deserve a fair share of the blame for that just for their having created and maintained the two-party duopoly along with the Republicans. At the very least its not the voters fault if the only viable alternative to the Republicans is constantly rigging (or in this case straight-up bypassing) their own primaries to put corrupt party insiders at the forefront.
Sure, everyone deserves some share of the blame, but it's like 10% for the Ds and 90% Rs. We can't keep talking like it's 50/50, that's how people become completely disenchanted with politics and don't even bother to vote.
The root cause is the first-past-the-post voting system that ensures a two-party system.
But for the party with the most responsibility for blowing it all up I'd like to nominate Rupert Murdoch. Most visibly with Fox News, but really his entire media empire