Conservative MPs are trying to force a showdown between a Liberal cabinet minister and his colleagues on the electoral reform committee, in a bid to draw attention to their push for a referendum on any change to how Canadians elect their government.

Scott Reid, the Conservatives' democratic reform critic, said Tuesday that the committee studying electoral reform options invited Foreign Affairs Minister Stéphane Dion to appear as a witness, but Dion declined. Reid moved to have a small group of MPs investigate whether Dion was "inappropriately or unjustifiably impeded in his right to be heard" at the committee, and, if so, who blocked him. The committee voted in favour of having its subcommittee look into the matter, though it can't force him to answer questions as MPs can't be compelled to appear.

The Conservatives want any potential electoral reform put to a referendum. They argue changing the system used to select Canada's federal government is such a major decision that all Canadians should have a say. Democratic Institutions Minister Maryam Monsef told the committee earlier this month that she's not convinced a referendum is the best way to decide a complex issue, but hasn't completely ruled it out.

The committee has until December to report to the House, at which point it would be a challenge to organize a national referendum to vote on a system Prime Minister Justin Trudeau has promised will be in place for the 2019 federal election. Reid has said he suspects the Liberal government is trying to run out the clock so there is no time left to hold a referendum.

Dion, who taught political science at the Université de Montréal for 11 years, wrote a paper published in 2012 that discussed the need for electoral reform and which alternate system would work best. The paper proposes a blended model of proportional representation and preferential vote.

In the conclusion, Dion wrote that "precedent makes holding a referendum necessary in Canada: changing the voting system would require popular support." He notes in the acknowledgements that the paper was published with then-Liberal Leader Bob Rae's consent, "but in no way does it constitute the Liberal Party of Canada's position."

While MPs voted to have the subcommittee discuss the issue, the NDP and Green members questioned the underlying assertion that the Liberals interfered with Dion's ability to appear before the committee.

"I just resist the suggestion that somehow someone as independently minded as Mr. Dion is being impeded by some force from testifying here," NDP House Leader Nathan Cullen said. "I'm sure if he wanted to come, he would have made that known to committee. I think there's more politics here than substance in this conversation."

Green Party Leader Elizabeth May said she'd like to hear Dion discuss the blended model he proposed in the 2012 paper.

"I don't see any conspiracies. If there's a way to see Mr. Dion testify, I'm all for it," she said. "He's entitled to decline... I don't think there's anything nefarious going on."

Conservative MP Jason Kenney argued Dion's appearance at committee would be in the public interest.

"A senior member of [Trudeau's] cabinet has repeatedly and publicly expressed himself as supporting referenda as being necessary for the legitimacy of any electoral reform... I think it would be interesting to hear from a senior minister whose view is different from that of the government."

A spokesman for Dion said it's his prerogative to decline a committee invitation.

"Minister Dion is actively engaged in his role as Minister of Foreign Affairs," Joseph Pickerill said in an email, listing meetings in Washington, D.C., as well as Asia. "The minister is occupied by representing Canada's interests and by global events. Minister Monsef has the full support of Cabinet."