On July 16, 2016, the RCP poll average had Hillary Clinton at 43.1 percent and Donald Trump at 40.4. At the moment, RCP has Biden at 48.7 and Trump at 40.1. The larger margin is obviously a problem for Trump — a 2.7 percent Democratic lead in national polls is compatible with a Republican electoral victory, but an 8.6 percent one (practically) isn’t — but Biden’s higher absolute level of support is also significant. He’s been above 50 in the average much more than Clinton was. A Trump comeback is probably going to have to involve not just winning on-the-fence voters but converting some current Biden supporters.