Corbett's lead reaches 15 points among likely voters, 47-32 percent, with 19 percent undecided.
Those poll results were similar to findings in a Muhlenberg College/Morning Call poll released yesterday. The results also reflect a slide for the Democrats, who last week enjoyed a round of polls showing Sestak neck-and-neck with Toomey and Onorato closing the gap on Corbett.
Sestak and Onorato, speaking yesterday to American Federation of State, County and Municipal Employees union retirees in Philadelphia, said their own internal campaign polls show them tied with Toomey and Corbett.
Both campaigns also pointed to a Reuters-Ipsos poll yesterday that showed Sestak tied with Toomey at 46 percent and Onorato trailing Corbett by just 6 points.
"We know we're in a statistical dead heat," Sestak said. "And every day we can feel the momentum moving us forward."
Toomey and Corbett were careful yesterday not to brag about the poll results, probably since Democrats enjoyed a surge just last week. Both said they were not focusing on polls.
Onorato yesterday complained that polls have fluctuated "all over the place" and denied that Corbett has a double-digit lead.
Corbett spokesman Kevin Harley said, "Tom Corbett is running this race as if he's 5 points down in the poll, regardless of whether the poll shows him 15 points up."
G. Terry Madonna, director of the Franklin & Marshall College poll, said recent polls are moving in the wrong direction for Sestak and Onorato, pulled down by high disapproval ratings among state voters for President Obama and Gov. Rendell mixed with serious concerns about the economy.
The recent bump for the Democrats in polls reflected stronger support within their own party, Madonna said, noting that Republicans say they are far more likely to vote than Democrats.
That means the Democrats must win the battle to turn out voters, especially in southeastern Pennsylvania, if they hope to counter Republican enthusiasm. Obama plans another visit to Philadelphia on Saturday, followed by first lady Michelle Obama on Monday. Madonna said the poll shows voter turnout is likely to be strong in the "heartland" part of the state, a challenge for Democrats.
"I think its really an uphill fight for Onorato," Madonna said. "Sestak you can't completely rule out."