Gerry Adams has accused Fianna Fail of dirty smear tactics over demands for him to admit his alleged IRA past.
The Sinn Fein leader, who is contesting a Dail seat in Louth, has always insisted he was never a member of the organisation.
But his Fianna Fail counterpart Micheal Martin said the persistent denials had cast a shadow over Mr Adams' credibility.
"There is a fundamental problem for Gerry when he continues to deny his membership of the IRA," said Mr Martin.
"Because every time he talks in this debate during the election about honesty, about being up front and transparent, it jars very much with his own position about the past.
"Martin McGuinness doesn't have a problem admitting his membership in the past but I think there is a huge problem for Gerry Adams in a credibility sense."
Opinion polls show Fianna Fail and Sinn Fein are slogging it out with each other for third place in next week's general election.
Mr Martin went on the attack after Mr Adams brought up white collar crime and fraud in Ireland during a five-way televised party leaders' debate on Monday night.
The Fianna Fail leader claimed Mr Adams and the Northern Ireland Deputy First Minister Martin McGuinness knew about plans for the infamous Northern Bank robbery during negotiations with the Irish and British governments.
But Mr Adams dismissed the allegations as a down and dirty ploy to undermine his party's neck-and-neck position with Fianna Fail. "This is a smear," the former West Belfast MP said.