Smith, a former coal company owner from Armstrong County, has more money than any other candidate so far after lending his campaign $4.3 million.
WPHT-AM host Dom Giordano, who hosted the debate, called their former Democratic registrations "the elephant in the room."
Smith, who was a Democrat for four decades, including while serving as a Plumcreek Township supervisor in the 1970s and 1980s, said he joined the party to honor his parents. He was a Democratic committeeman as late as 2010.
"It's true I was a Democrat, but I was conservative, so I really wasn't a Democrat," Smith explained, adding that he chaired a county tea party organization.
Welch, who voted for President Obama in the 2008 Democratic primary election, said he had gone to "painstaking lengths" to be clear about his political history. He renounced his support of Obama, while noting that Smith raised taxes during his time as a township supervisor and accusing him of trying to hide his political history.
Bucks County businessman David Christian joined the fray by noting that a company Welch owned had sought a $600,000 state loan while Gov. Ed Rendell was in office.
Welch brushed off the claim, saying that an employee had applied for the loan but that the company never followed through.
Former State Rep. Sam Rohrer of Berks County, who served nine terms before running for governor against Corbett in the 2010 primary election, said the April 24 primary was about who would "bear the conservative standard" against Casey.
Smith shot back, noting that he had supported Rohrer with campaign contributions.
"I'm glad you thought enough of me before I became a Republican to cash my check," Smith said, drawing laughs from the crowd.
Smith pressed forward on an issue that dogged Rohrer in his bid for governor: his 2005 vote for a legislative pay raise.
Rohrer, who rejected Smith's characterization of a "midnight pay raise," said the legislation covered salaries for many parts of state government and noted that he later sponsored legislation that repealed the raises for lawmakers.
Only Harrisburg lawyer Marc Scaringi, a former staffer to former U.S. Sen. Rick Santorum, stayed out of the fight about conservative credentials. He said the primary winner would "need to appeal to like-minded Democrats" to defeat Casey in the general election.
With less than three weeks to go until the primary, all five candidates face a daunting task in introducing themselves to voters. A Franklin and Marshall College Poll of 505 registered Republican voters released last week found that four out of five did not know enough about the candidates to say whom they would support.
Smith, who is already running campaign commercials, had 9 percent of the support, while Rohrer, who still has name recognition from his 2010 campaign, was at 7 percent. Welch, Christian, and Scaringi barely registered at 1 percent.
Contact Chris Brennan at 215-854-5973 or email@example.com, or follow him on Twitter @chrisbrenndn. Read his blog, PhillyClout.com