Bribery seems like the kind of crime where it takes two to tango.

If one person is charged with accepting a bribe, isn't it logical someone else should be charged with offering the bribe?

That's why many Canadians are scratching their heads about why Sen. Mike Duffy has been left without a dance partner, facing the music all alone.

Bribery is one of the 31 criminal charges the 69-year-old journalist-turned-politician faces in connection with questionable living expenses he billed to the Senate between 2009 and 2012. The Old Duff pleaded not guilty to all the charges in April when his trial began in Ottawa.

Nigel Wright, the former chief of staff to Prime Minister Stephen Harper, has not faced police accusations of criminal wrongdoing.

He testified a week ago he thought the RCMP might have been looking at him in 2013 for his role in Duffy's scandal.

The NDP piled on to Duffy's trial and Harper's misery Tuesday. The party's ethics critic Charlie Angus wrote to RCMP Commissioner Bob Paulson, asking the Mounties to consider charges against Wright, given information that has come out while Wright has been on the witness stand.

Liberal Leader Justin Trudeau also tried to capitalize on Wright's testimony, writing an open letter to Harper saying there were "extremely serious questions about your judgment and your public statements."

When news stories broke in 2012 about Duffy's dubious expense claims, it fell to Wright to put a lid on the emerging scandal.

In 2013, Duffy accepted a $90,000 payment from Wright to repay controversial living expenses Duffy racked up over four years. The RCMP considered it a bribe for Duffy to take it.

Despite his $140,000-a-year post in Parliament's upper house, Duffy said he didn't have the money to repay his expenses.

Wright nearly had a deal to have the Conservative Fund pay back Duffy's expenses. But when the amount was calculated at $90,000 instead of the $32,000 original estimate, the Conservative Fund walked away. So Wright reimbursed the public himself for what he considered Duffy's "inappropriate" expenses and tried to help the government out of a sticky situation.

Police and prosecutors routinely assess the prospect of getting a conviction before they lay criminal charges. There are at least four reasons why Wright won't get charged with bribery:

n He tried to do the right thing. Wright considered Duffy's expenses "an outrage on the taxpayers," we have heard. He was hell-bent to have them repaid, ideally by Duffy. But if not the senator himself, by someone.

n Wright helped out the RCMP. Before he cleaned out his desk at the Prime Minister's Office in May 2013, Wright compiled all the emails he wrote or received about Duffy's expenses. He packaged them up and turned them over to Parliament's ethics commissioner and to the RCMP.

n He'd be a sympathetic defendant. Wright's a rich guy who took the Prime Minister's chief of staff job, seemingly out of a sense of duty to the Conservative party and a noble belief in public service.

He was also caught between a rock and hard place. His boss -- the Prime Minister of Canada -- told him to take care of Duffy's expense scandal and prevent it from embarrassing the government.

n Wright was lucky enough to have the means to repay Duffy's expenses. There were audible gasps in the courtroom when the former managing director of the Onex Corporation said he could write a $90,000 cheque without significantly affecting his net worth. He considered it a "good deed." It would be hard to prove the criminal intent needed to get a bribery conviction against Wright.

No one is surprised the NDP and Liberals, in a tight, three-way campaign, are making political hay at Duffy's and Wright's expense.

But don't be surprised when the NDP's plea to the Mounties falls on deaf ears, Nigel Wright walks away and Duffy's the only guy left on the dance floor.

Twitter: @Corey_Larocque