Smith, a former coal-company owner from Armstrong County, has more money than any other candidate so far after loaning his campaign $4.3 million.
WPHT-AM radio's Dom Giordano, who hosted the debate, called their former Democratic registrations "the elephant in the room."
Smith, who was a Democrat for four decades while serving as a township supervisor in the 1970s and '80s, 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, said he has 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 an employee had applied for the loan but that the company never followed through on it.
Former state Rep. Sam Rohrer, who served nine terms before running for governor against Corbett in the 2010 primary, said the April 24 primary is about who will "bear the conservative standard" against Casey.
Smith shot back, noting that he has supported Rohrer with campaign contributions in the past.
"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legislation covered salaries for many parts of the state government and noted that he later sponsored legislation that repealed the raises for lawmakers.
Only Harrisburg attorney Marc Scaringi, a onetime staffer to former U.S. Sen Rick Santorum, stayed out of the fight about conservative credentials. He noted that the primary winner will "need to appeal to like-minded Democrats" to defeat Casey in the general election.
With fewer than three weeks to go until the primary, all five candidates face the daunting task of introducing themselves to voters. A Franklin & 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 who 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 firstname.lastname@example.org, or follow him on Twitter @chrisbrenndn. Read his blog PhillyClout.com