Prime Minister Stephen Harper says it's "regular practice" to assist members of the Conservative caucus with legal expenses.

Speaking one day after Sen. Mike Duffy delivered another bombshell address in the Senate -- alleging that the Conservative Party not only paid him more than $90,000 for his disputed expenses, but that they also covered more than $13,000 in associated legal fees -- Harper was hammered with questions from the opposition about how much he knew of the deal worked out between Duffy and his former chief of staff Nigel Wright.

Asked by NDP Leader Thomas Mulcair about the decision to reimburse Duffy's legal expenses, Harper said: "That is a regular practice. The party regularly reimburses members of its caucus for valid legal expense. As do other parties."

During question period Tuesday, a visibly frustrated Harper repeatedly stressed that Duffy accepted money for ineligible expenses that he has yet to repay, which he said was the "real issue."

"The reality is Mr. Duffy still hasn't paid a cent back to the taxpayers of Canada. The fact that he hasn't, and the fact that he shows absolutely no regret for his actions, and the fact that he's told untruths about his actions, mean that he should be removed from the public payroll," he said.

Asked to clarify whether Wright resigned or was dismissed over the $90,000 payment he made to Duffy to cover ineligible housing expenses, Harper said: "Mr. Wright did things that were inappropriate. That is why I explained this clearly to him: he could no longer work for me."

Liberal Leader Justin Trudeau asked Harper a number of times whether he would testify under oath about the Senate expense scandal and the alleged efforts by the PMO to cover it up.

However, Harper deflected the questions.

"The case here is, as we've said repeatedly, is that Mr. Duffy took $90,000 from the taxpayers that he should have not taken," he said.

NDP Deputy Leader Megan Leslie said later Tuesday that although Duffy has made some “wild claims,” he is the only one who has put forward evidence to back up his claims.

“We keep asking the prime minister, ‘Show us something, give us some evidence,’ Leslie told CTV’s Power Play.

“It’s hard not to believe the prime minister if he or she stands up in the House of Commons and says what is supposed to be the truth, but we have no evidence and yet we’re getting evidence from Duffy. So I don’t understand why he’s not coming clean on this.”

Liberal MP Rodger Cuzner accused the prime minister of “building a firewall around the truth,” and said he’s particularly troubled by the fact that paying Duffy’s legal fees was “part of the deal.”

“This was a contrived part of a deal to make the problem go away, to evade the truth and any kind of transparency around the issue, and I think that’s what’s most concerning,” Cuzner told Power Play.

CTV’s Ottawa Bureau Chief Robert Fife reported Tuesday that a drawn-out debate in the Senate on the suspensions of Wallin, Duffy and Brazeau is putting Harper in a tough spot ahead of Thursday’s Conservative Party convention kickoff in Calgary.

Amendments to suspension motions

Senators debated until around 11 p.m. ET Tuesday on motions calling for the suspensions of the three senators currently embroiled in the ongoing expenses scandal: Duffy, and Senators Pamela Wallin and Patrick Brazeau.

Conservative Sen. Yonah Martin, deputy leader of the government in the Senate, tried to introduce amendments to the suspension motions that would allow the three senators to keep their life insurance and medical benefits, according to Sen. Claude Carignan.

However, Martin failed to get unanimous consent to have the amendments heard.

It’s unclear if the amendments would sway more senators towards voting for the suspensions, since they fail to address one of the main concerns raised by the Liberals and some Conservative senators, including Sen. Hugh Segal: the right to a fair hearing and due process.