Here are the 538 polls for New Hampshire and Iowa. What I see is Pete is taking Warren’s support – when she’s up, he’s down, and vice versa.
Click on them to go to the originals on 538 if they’re too small. If people choose a candidate on ideology, then Warren losing voters to Pete makes no sense. But I think what we’re seeing is a choice based on perceptions of electability. Also, I realize that these graphs, and polls, don’t tell the whole story, and that there may be a phenomenon like Pete sucking up all the supporters of the candidates who dropped out (I doubt it). But it’s the best we have, and I thought it was interesting.