There are now 22 empty seats in the 105-seat chamber and Harper has signalled he has no intention of filling them — or any others that arise — in the foreseeable future.
He has not appointed a senator since March 2013.
In July, the prime minister formalized his practice of refusing to fill Senate vacancies, announcing a moratorium on appointments.
Yet, the federal government is in court arguing that the prime minister has made no decision to leave Senate seats unfilled.
The court case was filed by Vancouver lawyer Aniz Alani who believes it's unconstitutional to allow Senate vacancies to pile up indefinitely.
The Constitution stipulates that the governor general "shall" fill vacancies in the Senate when they occur. By convention, the governor general acts only on the advice of the prime minister.
As part of the court case, Alani requested that the Prime Minister's Office provide a record of "all materials" considered by the prime minister and bureaucrats in the Privy Council Office "in making the decision not to advise the Governor General to fill the currently existing vacancies."
In response, federal lawyer Jan Brongers wrote the court: "The respondents advise that there was no 'decision not to advise the Governor General to fill the currently existing (Senate) vacancies' as alleged by Mr. Alani."
Accordingly, Brongers adds, there will be no material provided to the court.
Brongers' letter is dated June 15, a little more than a month before Harper formally announced his moratorium on Senate appointments.
But it had been clear long before then that Harper had made a deliberate choice to let the empty seats go unfilled.
As far back as August 2013, Harper made it clear he had no interest in appointing senators so long as government legislation passed by the House of Commons continued to make it through the Senate.
“Obviously we’ll keep an eye on whether the legislation passed by the elected house is able to keep moving. As long as it is, I have no immediate plans," he said then.
Last December, Harper noted there was no public demand for more senators.
"I don't think I'm getting a lot of call from Canadians to name more senators right about now," he said.
"We're able to continue to pass our legislation through the Senate, so from our standpoint the Senate of Canada is continuing to fulfil its functions."
When he formally announced his moratorium on Senate appointments in July, Harper reiterated that it will apply only as long as the Conservatives have the numbers in the Senate to ensure passage of government legislation, which he said "should not be a problem for several years."
Alani is asking the Federal Court to declare that the prime minister has a constitutional duty to fill Senate vacancies within a reasonable time.
The federal government is in the process of appealing the court's refusal to dismiss the case outright.
Also on HuffPost