That Biden let into the US somewhere between 5 and 15 million illegals, many under some tenuous program of "parole" or such. That that amounted to open borders, and it was deliberate. Of course, I don't think Biden himself was the key actor here. I think radical Leftists on his staff were largely the key to that.
The result was that the population of illegals in the US roughly doubled in Biden's term and the results of that were so obvious, negative, and disgusting that people turned massively against the Democrats. It wasn't the only reason, but it was a major one. The Left were utterly blindsided by this in that they thought people would either not care or be welcoming of these "immigrants."
Now, it has become a major selling point for the Republicans, and the Democrats are shooting themselves not in the foot, but in the head, with a cannon trying to stop Trump from deporting the hundreds of thousands of criminals, potential terrorists, scum, and assorted villainy that Biden imported.