The race for Pennsylvania governor has tightened a smidgen entering the final six days, with Democrat Tom Wolf still enjoying a 13-percentage-point lead over Gov. Tom Corbett (R) among likely voters, according to a Franklin & Marshall College Poll released Wednesday.

Wolf has the support of 53 percent of voters who said they are very interested in the race and certain to vote, to 40 percent for Corbett, the poll found. Five percent said they "don't know" whom they will back next Tuesday.

Disaffected Republicans are "coming home" and coalescing around Corbett, a month-to-month comparison of F&M polls shows. His job-approval among GOP voters increased from 39 percent in August to 56 percent in this latest survey. The percentage of Republicans saying Corbett deserved a second term increased from 46 percent to 63 percent over the same period.

"Corbett has been able to make the case that Wolf is a tax-and-spend liberal," pollster Terry Madonna said. "That is a huge part of the momentum we're seeing."

The governor has been pounding Wolf in television ads and on the stump for an income-tax shift proposal that would increase the burden for the wealthy and give the middle-class a tax break. A lack of details has enabled Corbett to argue that Wolf would increase people's taxes.

In addition, "we've seen a sharper, crisper, more on-message Corbett than we've seen in the last three and a half years," Madonna said. The governor dominated at least two of the three televised debates in the campaign, the pollster said.

F&M tested three models of voter turnout. For instance, Wolf held a narrower lead of 50 percent to 43 percent among those voters with a record of casting ballots in four of the last five general elections, who also said they were very interested in the campaign and were "certain" to vote.

That scenario reflects a potential turnout on the low end Nov. 4, which many analysts expect. In general, Republican voters cast ballots at a greater rate in midterm elections.

Among registered voters, a larger universe that would represent a turnout closer to that of presidential years, Wolf led 50 percent to 32 percent. The self-described likely voters sample is based on a projected turnout between the two.

Pollsters interviewed 738 voters by telephone from Oct. 20 to 26, with a sampling error margin of plus or minus 3.4 percentage points.