The statewide survey by Franklin & Marshall College during the week that ended Monday found Toomey favored by 40 percent of likely voters to Sestak's 31 percent. About a quarter were undecided 10 weeks before the general election.
Toomey, a former congressman, and U.S. Rep. Sestak are vying for the seat currently held by Republican-turned-Democrat Arlen Specter, who lost his new party's nomination to Sestak in the primary.
In the gubernatorial sweepstakes, Corbett, the state attorney general, held an 11-point lead over Onorato, the elected chief executive of Allegheny County. Corbett was backed by 38 percent of likely voters, and Onorato by 27 percent. About one-third of those voters said they had not made up their minds.
Gov. Ed Rendell, a Democrat, must step down in January after serving the maximum two four-year terms allowed by law.
Among a larger group of registered voters, the candidates in both the Senate and governor races were running about even, with two out of five voters undecided in both contests.
G. Terry Madonna, a professor who oversees the Franklin & Marshall poll, said the results of the latest survey show that Democrats, who currently hold the Senate seat and the governorship, need to energize their voters to turn out on Election Day.
"The larger the turnout, the better the Democrats will do," Madonna said.
The horse race results were based on responses from Pennsylvania voters who said they were likely to go to the polls on Nov. 2. The sampling error margin for those results was a relatively high plus or minus 5.4 percentage points, reflecting the small number - 377 - of voters in that group.
Responses from the 485 registered voters carried a sampling error margin of plus or minus 4.4 percentage points.
The telephone poll was commissioned by 6abc, the Philadelphia Daily News, the Pittsburgh Tribune-Review, Times-Shamrock Newspapers in Scranton, The Patriot-News in Harrisburg, WGAL-TV in Lancaster and WTAE-TV in Pittsburgh.