There is no defense of Biden by the Dems.
There doesn't need to be, because it won't be the issue for the substantial group of independent voters who don't tune into politics until the last month or two before the election and don't get their news from conservative sources. All they're going to hear about is 1/6, "threats to democracy," and felony charges against Trump. And this assumes Biden is the nominee.
As we know, the Democrats could substitute another candidate who at least says he won't be insane on the border, and he'll just say, "I'm not Biden." But all the Trump negatives will still be there.
Of course, it didn't have be this way, but our primary voters decided to choose the hardest option with the least reward. We're paying to buy into Berkshire Hathaway, but we're getting stock in Cosmo Kramer's rollout tie dispenser company.