No it's not a "third of americans" It's a third of religious nutjobs who took the poll.
More democrats took the poll than republicans, and the largest religious group was "unaffiliated".
Survey designers who want useful results work pretty hard to correct for bias and capture the large-scale opinion (I can't say anything in this case other than "from a superficial view it seems like they did most things not incorrectly")
> “One-third of Americans (34%) say that immigrants entering the country illegally today are ‘poisoning the blood of our country’, including six in 10 Republicans (61%), 30% of independents, and only 13% of Democrats,” a summary of the annual poll stated, which surveyed more than 5,000 individuals from 16 August to 4 September.
A little under _two_ thirds of Republicans believe it, but a third of Americans overall.