The Quinnipiac University poll released Tuesday shows Corbett with 54 percent to Onorato's 39 percent among likely voters.
It is the Connecticut school's first survey in the race since July, when Corbett led 44 percent to Onorato's 37 percent among registered voters.
In the new poll, just 7 percent say they're undecided while independents favor Corbett almost two-to-one over Onorato.
The telephone poll of 684 Pennsylvania voters deemed likely to vote was conducted during the five days that ended Sunday. The sampling error margin is plus or minus 3.8 percentage points.
The election is Nov. 2.