Results of the Quinnipiac University poll released Monday indicated Clinton and Christie are tied at 41 percent.
Neither the Democrat Clinton nor Republican Christie has announced plans to seek their parties' presidential nomination.
Iowa voters said they had a favorable view of Clinton, 52 percent to 41 percent, said the Quinnipiac University Polling Institute in Hamden, Conn.
Christie has a 42 percent-to-16 percent favorability rating, with 40 percent saying they don't know enough about him to form an opinion, the poll indicated.
Biden drew a more negative rating, 38 percent favorable to 47 percent unfavorable. Sixty-five percent said they didn't know enough about Walker to form an opinion, the survey indicated.
Results are based on telephone interviews with 1,256 registered voters July 15-17. The margin of error is 2.8 percentage points.
Notable deaths of 2014 [PHOTOS]
SUNY Canton cancels classes over threat