Although Warren draws fairly broadly from different candidates' coalitions, Buttigieg supporters are more likely to have Warren as their 2nd choice than any other major candidate. projects.economist.com/democratic-pri… 
So if pro-Warren folks instinctually see Buttigieg as a threat, that instinct is correct, I think. (He could also complicate her path in Iowa.) Folks who are really into Warren's lefty policy ideas aren't gonna love Pete, but not that many people vote on the basis of ideology.
My colleague @baseballot had a bit more on this dynamic in a story today: 53eig.ht/36bQzoq
