A $5-million summer onslaught of negative television attack ads has not improved the standing of Pennsylvania Gov. Tom Corbett in his run for a second term, according to a new poll released Thursday.

Democrat Tom Wolf holds a lead of 25 percentage points in the Franklin & Marshall College Poll, 49 percent to 24 percent, with 25 percent undecided. That has budged little from F & M's last survey in June.

Just over a quarter of respondents, or 26 percent, said that Corbett has done a good enough job to have earned reelection, the poll found.

About 8 in 10 voters have seen television spots for both candidates.

Corbett has run ads ripping Wolf for running a kitchen-cabinet company that is registered in Delaware, charging that he is taking advantage of a loophole to avoid Pennsylvania corporate taxes – though the governor has offered no proof. Wolf and the company have denied using the loophole, but have declined to release corporate tax records that could refute Corbett's attack.

Other ads have seized on Wolf's desire to change the state's flat income-tax in order to tie the rate to income, so that wealthier taxpayers would pay more. Corbett's campaign argues that the Wolf plan could hike the tax burden for households making as little as $70,000, which many consider a middle-class income.

The poll is based on live telephone interviews with 520 registered Pennsylvania voters, conducted between Aug. 18 and Monday. Results are subject to a sampling-error margin of plus or minus 4.3 percentage points.