Obama leads with 51 percent support over Romney at 39 percent. Obama had only a 4-percentage-point advantage, 48 to 44, in the same poll from mid-July.

The 12-point advantage is the biggest lead Obama has had in Pennsylvania this year. The president leads Romney by 9 in Pennsylvania, according to the Real Clear Politics average of polls.

Pennsylvania is one of 12 key swing states that President Bush won in 2004 but Obama won in 2008. The state’s 20 electoral votes are the third-most at stake among swing states, behind only Florida and Ohio. 

Obama won Pennsylvania by 11 percentage points over Sen. John McCainJohn Sidney McCainTrump's dangerous Guantánamo fixation will fuel fire for terrorists Tech beefs up lobbying amid Russia scrutiny Ad encourages GOP senator to vote 'no' on tax bill MORE (R-Ariz.) in 2008.

The survey of 500 likely voters was conducted on Sept. 19 and has a 4.5 percentage point margin of error.