Ok, if you don't like polls from Pew, Kaiser, random universities, etc.... Consider this.
There were two socialists on the Democratic ticket in the primary. Elizabeth Warren and Bernie Sanders.
Roughly 13 million Democrats voted for socialist candidates in the primaries. Biden got 18 million votes. Also consider the policies of Bernie and Warren made their way into the official platform of the party, some to a lesser extent. Biden went with an option to buy into Medicare, a half option on tuition free college, etc. But he had to shift or lose to socialist candidates, because the voters in the party want the policies.
In Kentucky.... Progressive candidate Charles Booker raised $3 million to challenge Amy McGrath who raised $40 million and had the support of the entire mainstream Democratic establishment. He lost to her by 2 points in their primary election. 247,000 votes to 232,000 votes. He was running on Medicare For All, Green New Deal, etc. And the voters nearly overcame a 1,000% money disparity in KENTUCKY.
Another very liberal candidate, Beto O'Rourke... Supports people buying into Medicare. Wants to ban military style weapons. Wants Universal prek for kids and families... Wants a huge increase in the minimum wage... wants to ban the death penalty... Supports wealth taxes... Wants to tax corporations and the richest americans over a 10 year period of time to fund our move to 100% green energy...
He ran against the 2nd most popular Republican in the nation, Ted Cruz, in the state of Texas. 4,045,632 Texans voted for him. 4,260,553 voted for Ted Cruz. 50 to 48. 10 years ago, 20 years ago, etc that would be UNTHINKABLE.
You can doubt the polls all you want (they aren't wrong, but whatever).... but the shift in voter demographic is obvious to anyone paying attention.