The "radical left" stayed home in the presidential election, 9 million of them decided Harris wasn't radical enough for them so they would rather not vote, giving Trump the 2.3 million vote popular margin. Of course electoral analysis would be a lot more complex, I'm not doing that.
Elected democrats are stuck between trying to appease the radical left and trying to actually govern along with republicans and those two are very incompatible goals because the radical left knows very little about actual government policy and just has a couple of very narrow issues that most of the country opposes that their social bubble convinces them are the only important issues in this country.
Democrats lost the last election because of the radicals AND didn't get any of their goals done. Democrats needed a more centrist charismatic leader and instead they keep nominating candidates who "deserve" the nomination opposed to the actual will of the people OR continue nominating ancient relics who needed to retire a decade earlier... 6 of whom died in office over a period of 13 months in a period where every vote counted.
In short, I blame stupid leftist radicals and corrupt self-interested Democratic cowards in office for our current situation.