Despite Nintex's hard hitting analysis, Warren hasn't really taken too much from Biden I don't think as much as she's eaten up a lot of the undecideds. Looking at the RCP average Warren is at 26.6, Biden at 26.4. Biden has been lower at times in the past, although they were short lived and he moved back up towards around a 28-30 average usually. Sanders has collapsed like a heart attack victim from his 16-17 average down to 14.6. And Harris has been vaporized off the planet down to 4.4% where she had been like 6-7% forever. Tulsi, Booker, Beto and Castro have all shed their point or so bumps, and Buttigieg's small bump has only restored him back to near his 6% average. Warren seems to have gobbled up all of these. Warren ate from the earlier Biden decline more than the others too.
The big Harris bump that disappeared seems to have gone almost entirely to Warren, I say this not from analyzing the numbers but looking at the graph, it seems as if they became undecideds/Bidendefaults until recently when they've flocked to Warren:
They probably did not go to Bernie because of a reflexive reaction towards his skin-crawling weirdness, virulent racism and violent sexism.
