Bernie Sanders is, of course, the quintessential example, as he polled better against Trump than all 3 of Trump's eventual Democratic opponents. Katie Porter flipped a red district and was well known for taking corporate stooges to task; the DNC undermined her latest election, and now she's out of politics, IIRC.
Then there's the case in Kentucky, where Charles Booker had a real chance to unseat Mitch McConnell in 2020; he was exceptionally charismatic and had poll numbers that were rising terrifically fast because he was home-grown and made a point of trying to unite people through shared interest. The DNC shoveled millions into primary opponent Amy McGrath's campaign, and even locked black Kentuckians out of their sole voting center in Louisville, suppressing the vote; right-of-center McGrath won, but it's hard to overstate by how narrowly.
She was trounced in the general, and it's important to point out why: because she represented too little difference from McConnell. She was never going to peel voters off the real thing with a milquetoast knock-off. Booker growth in the polls before it was cut short was so pronounced becaus he offered a real choice to Kentuckians. But the problem, for the DNC, isn't that far-left policies aren't popular (they are, wildly, and particularly among the demos that stay home if not activated with a promise of positive change); it's that those policies are anathema to the elite within the party and party donors.
That's the actual reality. Which is sobering, because it means that the left's best chance to make real progress would be when an economic reckoning robs that elite of the funding to buy their preferred candidates.