Hanabusa leads Schatz with 50 percent support to 42 percent in a live-caller poll, conducted for the Honolulu Star-Advertiser and Hawaii News Now by Ward Research.
Schatz has raised and spent more money on the race so far, but both candidates have been on the air for weeks heading into the primary.
Their allies have been as well: The League of Conservation Voters has been on the air for Schatz, while EMILY's List has been airing spots for Hanabusa.
The poll also shows Hawaii Gov. Neil Abercrombie (D) badly trailing his challenger, state Sen. David Ige (D), by 18 points. Abercrombie has trailed Ige in most polls, but this is a larger margin than other surveys have found.
The live-caller poll of 692 likely voters was conducted from July 21-29 and has a margin of error of plus or minus 4 percent.
This post was updated at 2:20 p.m.