Some conservatives grumble about Corbett's missteps and express concerns about his electability against a Democrat next year, but they also say they see no signs that Castor - or any other plausible GOP challenger - is taking serious steps to set up a campaign. Moreover, some question whether there is enough grassroots anger to fuel a successful insurgency.
Corbett remains shaky in public opinion polls and has made his share of missteps as a party leader.
He pushed a voter-ID law that was suspended amid legal challenges and that also inspired a turnout-boosting backlash among Democrats; he supported legislation to allocate the state's electoral votes by congressional district, angering fellow GOP leaders; and he strong-armed the party's state committee into endorsing his choice for U.S. Senate, Chester County venture capitalist Steve Welch, a former Democrat, who went on to finish third in last year's primary.
Still, no incumbent Pennsylvania governor who sought reelection has ever lost, and, as a rule, primary challenges just don't happen.
"The governor controls the mechanism of power and all the patronage and appointments," Castor said, acknowledging how difficult it would be to unseat Corbett. He argued that it would make sense to switch nominees if it looked as though the governor were doomed in next year's election, akin to 2006, when it seemed clear then-Sen. Rick Santorum (R., Pa.) was headed for defeat.
But GOP strategist Mark D. Harris said there was no "groundswell" behind Castor because he's an imperfect conservative messenger, having voted for tax increases in his county. And Castor and Corbett have some history - "everybody realizes it's not about ideology but personality," Harris said.
The two men have tangoed in a nasty primary before - in 2004, when Castor lost to Corbett in the race for state attorney general.
That year, Castor, then Montgomery County district attorney, lost the party endorsement to Corbett, a former U.S. attorney. But Castor stayed in the race, lacerating "corrupt party bosses."
He directed much of the fire at GOP powerbroker and funder Bob Asher, who backed Corbett. Ad after ad ripped Asher's federal conviction in a 1986 pay-to-play case, for which the Republican national committeeman spent eight months in prison. Castor also ran a spot calling Corbett "Forrest Dump" over his legal work for the waste industry.
Corbett, in turn, attacked Castor's campaign for taking a $600,000 contribution from ex-U.S. Transportation Secretary Drew Lewis, whom his office had prosecuted in 2002 for drunken driving.
"There's no personal angle to this," Castor said, noting that he was friendly with Corbett before 2004 and had backed him in races since. "I don't have anything bad to say about him as a man."
To be sure, Corbett is vulnerable. In a March 13 Quinnipiac University poll, he was under water, with 39 percent of registered voters who responded approving of his job performance, compared with 44 percent disapproving. Most of that was strong disapproval among Democrats and independents, though not all Republicans were wild about the job he has done: 67 percent approval to 27 percent disapproval.
To many conservative critics, Corbett has not taken the opportunity to push for such ideological priorities as a right-to-work law that allows employees to refuse to join a union, as was enacted recently in Michigan. Some want to see whether he will accept Medicaid funding from the Obama administration.
"Right now, all eyes are on Gov. Corbett to see whether he protects us from Medicaid expansion, and whether he will seriously go after the unions," said Jennifer Stefano, state director of the activist group Americans for Prosperity, which boasts 120,000 supporters.
Corbett declined federal money to expand the Medicaid rolls to cover uninsured people under the Affordable Care Act, a decision cheered by conservative activists who fear expanding the program will leave state taxpayers on the hook for massive costs later.
The governor shares those concerns but has been meeting with Obama administration officials and studying his options. Activists are "concerned he's going to flip his decision," Stefano said.
On the other hand, Corbett has slashed spending and held the line on taxes, and he recently earned a victory when the state House passed his plan to privatize the state liquor system, another favorite cause on the right. That "changes the equation" a bit, Castor conceded, though leaders of the GOP-controlled Senate have been cool to privatizing.
It's hard to build a campaign around unrealized goals such as right-to-work legislation, strategists say.
"I don't think anybody loses a primary because of things they don't do; people lose primaries because of things that they do, votes or decisions the base hates," Harris said. "As Republican primary voters become more aware of what he's done, they're going to be supportive of Tom Corbett."
Jeff Coleman, a conservative strategist, agreed. He knows something about challenging Corbett from the right - he ran former State Rep. Sam Rohrer's losing bid in the 2010 GOP gubernatorial primary.
Coleman credited Corbett with having pushed for antiabortion legislation and avoiding tax increases. He said the governor's oft-criticized stump skills were hardly enough grounds for his own party to upend him.
"The argument some seem to be making is based on sins of omission, not commission," the strategist said. "He's not walking away from core issues - it's about communication style, not anything that rises to a 'fireable' offense."
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