The formidable bar might also help explain why the other party in the transaction — Nigel Wright, the prime minister's former chief of staff — does not face a bribery charge.
The RCMP have laid 31 charges of fraud, breach of trust and bribery against Duffy. Many of them are connected to expense claims for housing and travel.
In Prince Edward Island, Duffy said Friday it would be inappropriate to comment because the matter is now before the courts.
"The court process will allow Canadians to hear all of the facts. They will then understand that I have not violated the Criminal Code," he said.
Duffy's first court appearance is set for Sept. 16.
The bribery charge, which carries a maximum sentence of 14 years, relates to a $90,000 payment Duffy received from Wright to allow him to repay his residency expense claims.
The RCMP said in April that Wright would not be charged.
Wright said at the time he intended to secure the repayment of taxpayer funds, and that he believed his actions were lawful and in the public interest.
Section 119 of the Criminal Code makes it an offence for a member of Parliament to "corruptly" accept money "in respect of anything done or omitted or to be done" in their official capacity.
Such cases are rare and the outcome of Duffy's bribery charge could turn could depend on how the courts interpret the wording, indicated Don Stuart, a law professor at Queen's University in Kingston, Ont.
"It seems unclear what the courts have made of the word corruption. Normally speaking you don't have to prove a motive, but in this case you might have to, because of the use of the word corruption," he said.
"They will have to prove beyond a reasonable doubt that his intention was a corrupt intention."
The lofty level of proof is due to the gravity of the offence — underscored by the stiff maximum penalty, said law professor Carissima Mathen of the University of Ottawa.
The word "corruptly" suggests an act done for a particular purpose, she said. "It's blameworthy, it's especially wrong what you've done."
Assuming that the RCMP is operating in good faith, this hurdle is the likely explanation for why Wright was not charged with bribery, Mathen said.
"We can find aspects of Wright's conduct problematic, troubling, even wrong — and it doesn't mean that in law the RCMP felt that there was enough evidence to present a reasonable chance of conviction," she said.
"In a legal sense, it is possible that the evidence was not there to show that Nigel Wright had the state of mind required for this crime. This is a serious crime."
The RCMP dropped some hints in previous court filings concerning why Wright might not be charged.
They noted that Wright had paid for tens of thousands of government business expenses out of his own pocket, in keeping with his personal philosophy. There was also reference to thousands of dollars in legal work he had done on his own dime.
For the RCMP, it might have reinforced the notion of no corrupt intent on Wright's part.
RCMP Commissioner Bob Paulson has said the Mounties have prepared an explanation as to why Wright was not charged, but how or when it will be made public is not clear.
— With a file from Jennifer Ditchburn
Follow @JimBronskill on Twitter
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