My impression of Trump is that he is a bar-room brawler on the campaign trail.
He hasn't laid a glove on Biden. Sure there have been a few jabs made in passing but nothing like the barrage he unleashed throughout the 2016 campaign on everybody between him and the White House.
Is he being strategic? Is he HOPING for Biden as an opponent and doesn't want to spook the Democrats into picking a less gaffe-prone candidate at the last moment?