After the Supreme Court of Canada ruled last year that his unilateral push for Senate reform wasn’t allowed, Prime Minister Stephen Harper might have been expected to take pains to stay within constitutional bounds in any future bid to overhaul the upper chamber. Harper had tried to move alone on imposing term limits and electing senators, but the court said he needed support from seven provinces with 50 per cent of the population. Harper seems undaunted, though. His new strategy is to refuse to name any new senators. And some experts say that’s not permitted under the Constitution, either.

Harper had already let 22 vacancies go unfilled in the 105-seat Senate before he announced, in a news conference last week with Saskatchewan Premier Brad Wall, that he will let attrition keep hollowing out the place as its patronage appointees die or retire. “It will force the provinces over time,” the Prime Minister said, “to either come up with a plan of comprehensive reform, or to conclude that the only way to deal with the status quo is abolition.”

Harper didn’t explain exactly why dwindling Senate ranks will alarm premiers so much that they will finally coalesce around a common reform or abolition plan. In fact, Harper even said senators don’t really do much now for their home provinces. It came up when he was asked if provincial governments might object to his refusal to fill vacancies from their turf. “The number of senators [that provinces] have today, in the current institution, gives no real weight in Parliament,” he said. “Decisions are made, for all practical purposes, in the House of Commons.”

Beyond the paradox in Harper’s position—senators are useless to provinces, yet premiers won’t tolerate empty Senate seats—are serious doubts that the Constitution permits this course of inaction. “I think that for him to refuse to appoint senators is manifestly unconstitutional—he can’t do that,” said University of Ottawa law professor Carissima Mathen. University of Waterloo political science professor Emmett Macfarlane, author of Governing from the Bench: The Supreme Court of Canada and the Judicial Role (and a Maclean’s contributor) agreed that naming new senators is “absolutely an obligation” for any prime minister.

That obligation is set out in the Constitution Act, where Section 32 says, “When a vacancy happens in the Senate by resignation, death, or otherwise, the Governor General shall . . . fill the vacancy.” By convention, of course, the Governor General acts only on the Prime Minister’s advice. Beyond this requirement, there’s the Senate’s constitutionally defined role, which includes representing provinces and regions and reviewing legislation passed by the House. If Harper leaves the Senate with too few members to do its job, Macfarlane says he will be “unilaterally impairing its essential function.” And that isn’t likely to survive a legal challenge.

But it will take a court, in the end, probably the Supreme Court, to make that ruling. Aniz Alani, a Vancouver lawyer, had already launched a case to try to force Harper to appoint senators before he made the moratorium an explicit policy. Mathen says judges might have been reluctant to come down firmly against the Prime Minister, if the case forced them to interpret what lies behind his reluctance to fill Senate vacancies and consider how quickly constitutional convention requires him to act. But, now that he has openly framed his refusal to name senators as a tactic for bringing provinces to the constitutional bargaining table, the facts that any court will confront are less ambiguous. “With his open avowal that he doesn’t think he has a duty to appoint, that’s where it gets tricky,” Mathen says. “Now, a province like Prince Edward Island or Manitoba can say, ‘He apparently has no intention of fulfilling our allotment under the Constitution.’ ”

Remarkably, Harper is not politically isolated in his willingness to just let the Senate wither. NDP Leader Thomas Mulcair has said that if his party wins this fall’s election, he would negotiate with the provinces to try to reach the unanimity needed to scrap the Senate—likely a long, hard slog—and would not appoint senators while he tried to win every single premier over to the NDP’s long-standing position. But the NDP has also suggested a more direct assault on the Senate’s ability to function: cutting off its funding. A motion tabled by Manitoba NDP MP Pat Martin to deny the Senate $57 million of its total 2015-16 budget of $88.7 million was voted down by Liberal and Tory MPs in the House early last month. “So Harper and Mulcair are on the same page now,” Macfarlane says. “We have a very rare situation, maybe unprecedented, where the Prime Minister and the official Opposition leader are advocating plainly unconstitutional action.”

That leaves Liberal Leader Justin Trudeau playing the only hand on Senate reform that stays safely within the constitutional rules of the game. Early last year, for starters, he kicked Liberal senators out of his parliamentary caucus. If he becomes PM, Trudeau vows that his Senate appointments would be guided by a new “independent-minded, non-partisan” panel’s recommendations. If his position doesn’t generate the I’m-fed-up heat of a moratorium on appointments or an abolition pledge, it has the advantage of being constitutionally allowed and politically doable. The question is whether Trudeau, now trailing at third in the polls, can turn a practical alternative into a campaign-trail asset.