The poll also found Brown would defeat any of the likely Republican candidates if the election were held now, LegalNewsline.com reported. Newsom was in a statistical tie with all of the Republicans.
The poll of 600 likely voters was done by Research 2000 for the Web site Daily Kos between Sunday and Wednesday. The margin of error is 4 percent either way.
Brown leads Newsom by 29 percent to 20 percent among those surveyed. A poll by J. Moore Methods in June gave Brown a 20-point lead.
Former eBay CEO Meg Whitman is the Republican front-runner with 24 percent of the vote to 19 percent for former U.S. Rep. Tom Campbell and 9 percent for state Insurance Commissioner Steve Poizner.
Two Republican consultants said Brown, a former governor, is likely to be the next one.
"He has as good or better odds of being elected than any other candidate in the race," said Dan Schnur, director of the Jesse M. Unruh Institute of Politics at the University of Southern California.