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ABC News' Michael Falcone reports:

CORNWALL, Pa. - According to the Republican governor of Pennsylvania, Mitt Romney's short-list of potential running mates can get a lot shorter.

Tom Corbett, the first-term governor of the Keystone State, cited only fellow state chief executives when asked on Saturday who he viewed as the top contenders to join the presidential ticket.

"I'm leaning towards the executives who had to make tough decisions," Corbett said in an interview. "Because I think where the president has stumbled when he came into office - he was never in that executive position where - it's not just a vote, it's the decision."

Corbett even named names.

"There are a lot of good candidates," he said, describing Gov. Mitch Daniels of Indiana as a "very good governor." He said Gov. Susanna Martinez of New Mexico "is doing a great job," and also praised Gov. Nikki Haley of South Carolina and Gov. Bob McDonnell of Virginia. (Corbett paid no lip service to other possible contenders like Ohio Sen. Rob Portman and Wisconsin Rep. Paul Ryan, who will campaign with Romney over the next two days, or to Florida Sen. Marco Rubio, who Romney often mentions in speeches.)

And, in case Romney was wondering, the Pennsylvania governor said he wasn't interested in the job.

"Not me," he chuckled. "I'm busy here."

A new Quinnipiac University poll of Pennsylvania voters released last week showed Corbett with his lowest approval ratings of his first term. Just 36 percent of Pennsylvanians said they approve of the job he is doing compared to 47 percent who disapprove.

Corbett introduced Romney at his third and final event of the day at the historic Cornwall Iron Furnace about 25 miles from the state capitol in Harrisburg. Romney visited Pennsylvania on the second day of his five-day, six-state bus tour through key election battlegrounds.

Confronted with recent polling that gives Obama a slight edge in Pennsylvania, the governor acknowledged that "this is a tough state" and "not an easy state for Republicans to win," but he added, "it is winnable."

Corbett said the biggest challenge Romney faces is a huge voter registration advantage for Democrats. But, he insisted, "it's surmountable."

"He's not going to win Philadelphia, we know that," Corbett said of the presumptive Republican nominee. "But he can compete in the rest of the state."

As Pennsylvanians filed out of Romney's event in Lebanon County, which John McCain won by nearly 20 percentage points over Barack Obama four years ago, Corbett voiced optimism for Romney's prospects in November.

"This is a very different dynamic this year than it was four years ago," he said. "You can feel it. You can absolutely feel it."