Prime Minister Stephen Harper is standing by his chief of staff but is so far avoiding questions as to how his most senior adviser became directly involved in a growing controversy over Senate expenses.

New details emerged Thursday as to the surprising transaction between Nigel Wright and former Conservative Senator Mike Duffy. Sources confirm the chief of staff sent a cheque to the senator's lawyer for $90,172 on March 25. Mr. Duffy then paid the Receiver-General the exact same amount that day. Officials said this week that the Prime Minister was not told of the arrangement.

There is also an increasing focus on Mr. Duffy's past work for the Conservative Party – including during the 2011 election campaign – as records suggest he was claiming expenses for Senate work on the same days that he was campaigning for the party.

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Ethics Commissioner Mary Dawson is looking into Mr. Wright's payment to the senator and the NDP is urging the Senate ethics officer to do the same. But the government line is solidly in support of Mr. Wright, who came to the PMO on a leave from a successful career on Bay Street.

"Mr. Wright will not be resigning," said Andrew MacDougall, Mr. Harper's director of communications. "Mr. Wright has the full support of the Prime Minister."

Mr. Harper was in New York Thursday, where he took audience questions from the Council of Foreign Relations on foreign affairs issues. He did not take questions from the media and has not yet commented on Mr. Wright's payment to Mr. Duffy.

Rob Walsh, who served for 12 years as the law clerk and parliamentary counsel until his retirement last year, said the government can't simply brush aside Mr. Wright's actions.

"It really is quite appalling and there should be a full accounting made by the Prime Minister's Office and by Mr. Duffy," he said. Mr. Walsh served as the non-partisan guide to Parliament in ethics-related investigations under both Liberal and Conservative governments. He said what makes the situation a serious matter is the suggestion that the Prime Minister's Office apparently interfered with the Senate investigation of Mr. Duffy's expenses.

"This looks like a cover-up," he said. "This sort of a transaction can't be written off as a mere private transaction. That's preposterous."

The RCMP would not say Thursday whether it is broadening its current review of Senate expenses to include the payment from Mr. Wright. Following last week's release of audits and Senate committee reports into the expenses of Mr. Duffy and Senators Patrick Brazeau and Mac Harb, the RCMP said they would be performing an "evaluation of the information provided," and may or may not conduct a full investigation.

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"It's not common practice to confirm or deny who or what is the subject of any investigation," said RCMP Corporal Lucy Shorey Thursday when asked for an update.

The audits of the three senators took place in response to accusations they were misusing an entitlement that allows living expenses to be claimed while working in Ottawa if a senator's primary residence is more than 100 kilometres from Parliament Hill.

On Feb. 22, Mr. Duffy announced that he and his wife decided to repay the housing allowance "rather than let this issue drag on." He did not disclose an amount at that time. However, Conservatives say Mr. Duffy then had difficulty delivering on the pledge and it was at that point that Mr. Wright stepped in.

CTV News reported earlier this week that the arrangement included a pledge that the government would go easy on the senator.

Mr. Brazeau – who is facing criminal charges for assault and sexual assault in a matter unrelated to the Senate – made a rare public appearance Thursday to criticize the process surrounding the review of expenses. He told CBC's Power and Politics that the committee ordering him to repay $48,744 owes him a public explanation, given that auditors reported to that same committee that he broke no rules and met all four criteria the Senate uses for determining residency requirements.