Biden and Bernie have each gained ~2 points over the past month, which ain't nothing, but also isn't that much. And their current standing is pretty consistent with where they've been on average throughout the campaign… 
…so the notion that Biden and/or Bernie are now making comebacks, which has become a common motif in campaign coverage, is semi-dubious. A big part of the story is they were somewhat ignored while the media chased shinier objects (e.g. Warren, Buttigieg, Harris, Bloomberg).
Trying not to make this too hindsight-is-20/20. There were reasons to be bullish on Harris. Warren and Buttigieg still have decent chances. The fluctuations in polls aren't super predictable. But Biden has been *leading the whole campaign* and rarely been covered like it.
