Using the latest batch of court filings to bolster his claim, Cowan will argue that the alleged interference "began virtually from inception," starting with a call purportedly made by Prime Minister Stephen Harper's then-chief of staff Nigel Wright to "a member of the [internal economy] committee" seeking changes to the press release announcing the referrals to Deloitte.
In the court documents, RCMP Cpl. Greg Horton alleges Wright and Duffy committed bribery, fraud and breach of trust when Wright wrote Duffy a cheque for more than $90,000 to cover Duffy's questionable Senate expenses.
Horton also alleges the Prime Minister's Office tried to interfere with an audit into Duffy's expenses, as well as with the Senate committee's report on the audit.
Cowan says the alleged interference started with the PMO and continued.
"The interference continued when Mr. Michael Runia, a managing partner with Deloitte, telephoned auditor Gary Timm allegedly at the request of the Prime Minister's Office, seeking information about the [Duffy] audit," Cowan contends in the letter sent to Senate Clerk Gary O'Brien earlier today.
"Consequently, this is my earliest opportunity to seek a remedy through a formal question of privilege to deal with this matter, now that all other reasonable avenues of redress have been blocked."
The latest move comes one day after the Conservative-controlled Upper House nixed a motion that would have ordered the Senate internal economy committee to question Runia about his involvement in the Duffy audit.
Cowan is expected to expand on his arguments when he rises in the Senate Chamber this afternoon.
In the meantime, read the full notice letter below. Mobile users, click here to view in your browser.