Justice Charles Vaillancourt will hear arguments Monday in what is called a voir dire, which is basically a mini-trial within the main event.
Lawyers will argue over the admissibility of a 2010 Senate report based on an audit of senators' office expenditures and service contracts .
Duffy's lawyer, Donald Bayne, wants to treat that report as fact and possible use it to prove his contention that the Senate knew its rules were vague and confusing.
The Crown, however, wants the report treated as opinion, not fact — a key distinction when the judge later reviews the evidence.
The trial is already expected to overrun its 41-day schedule by weeks and this pause will extend the proceedings even further.
On Wednesday, Tom Mulcair charged that an attempted coverup of Duffy's residency issues was all about the prime minister trying to protect his own reputation.
The Opposition leader was responding to suggestions this week that Harper overrode Duffy's concerns about being appointed to the Senate to represent P.E.I. in late 2008.
A source told The Canadian Press that Duffy suggested he represent Ontario, since that's where he had lived for decades, but Harper was insistent about P.E.I.
Bayne has argued during the trial that Duffy followed Senate rules as they exist and, if anything, is guilty of administrative errors and nothing criminal.
Also on HuffPost