Could the prime minister's former chief staff and several senior Tory senators wind up testifying at Sen. Mike Duffy's trial just as the federal election campaign gets underway?

This scenario is becoming increasingly likely, as the pieces move into place for extra trial dates to be set aside in August to hear Duffy's case.

Whether or not the election writ is officially issued, the parties will be in full campaign mode by that point — the Conservatives have already agreed to a leaders' debate in August.

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Last week, a lawyer for former senator Mac Harb agreed to postpone his trial scheduled to begin on Aug. 10, as Duffy's trial requires more time.

"Also we have some interest in some of the issues in the Duffy trial, so we'll see how some of those issues unfold," said Harb defence lawyer Sean May.

That means the Harb fraud and breach of trust trial will likely unfold in the spring of 2016, with more key witnesses appearing at Duffy's trial in the last three weeks of August.

Both Harb and Duffy have been charged with filing improper living expenses connected with their designation of an Ottawa-area home as a secondary residence.

Duffy has pleaded not guilty to 31 charges in total, including others that relate to his travel and Senate office expense claims.

After a three-week hiatus, the trial resumed Monday with a minor victory for the Duffy team. Justice Charles Vaillancourt ruled that a Senate committee report could be entered into evidence.

The 2010 report of the internal economy committee was based largely on three audits of the Senate's administrative practices, undertaken by the firm Ernst and Young.

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It found that there was a lack of clear guidelines and criteria establishing what constitutes official business, as well as what is considered partisan activity, when senators file expenses.

Those conclusions are important for Duffy's defence, which is largely focused on the argument that he was operating within the Senate's confusing and lax rules.

"Oversight is relevant in this trial and counsel for the applicant advances the proposition that the lack of appropriate oversight is a key component to the defence of many of the charges before the court," Vaillancourt wrote. "I recognize this as a valid position."

The Crown argued that the report should be considered hearsay, since it was based on the work of an audit firm and not the committee. But Vaillancourt agreed with the defence that the audits were absorbed into the committee's own work.

This ruling could have implications later on in the trial. The auditor general is set to submit to the Senate a report on the spending of all senators some time this week.

From that point, the report, its release to the public and the official response all moves out of the auditor's hands. Bayne can be expected to argue that it, too, is a Senate document and should be entered into evidence at Duffy's trial.

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Vaillancourt is also hearing a separate evidence issue that involves another internal Senate audit. The upper chamber is trying to keep it from being released to the defence, arguing parliamentary privilege or immunity. That matter will be considered next week.