On its face, Prime Minister Justin Trudeau’s decision to threaten Conservative Leader Andrew Scheer with a defamation lawsuit is needs-to-have-his-head-examined foolish. The threat adds fresh fuel to the SNC-Lavalin bonfire. The only question seems to be: What on earth were the Liberals thinking?

To understand that thinking, it may help to go back to June 16, 2015, a few months before the federal election that made him prime minister.

On that day, Mr. Trudeau held a press conference in which he unveiled a slew of election promises that included everything from electoral reform to restoring home mail delivery. These were desperate promises – those two, in particular, were never fulfilled – from a party that was in desperate shape: behind in the polls, lagging in fundraising and lacking a sense of purpose.

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Mr. Trudeau had won the leadership in 2013 after the Liberals expanded the voting pool beyond paid-up party members, weakening partisan ties among the grassroots. The party was already weakened by internal conflict and repeated electoral defeat. Mr. Trudeau and his advisers made it clear to most veterans of the Chrétien, Martin, Dion and Ignatieff years – and wars – that their counsel was neither welcomed nor required. He expelled Liberal senators from caucus. The party became the leader and little else.

Open this photo in gallery Liberals may believe that since the SNC-Lavalin scandal isn’t going away any time soon, the threatened lawsuit will at least allow supporters to claim Mr. Trudeau is innocent and that he is being defamed by his opponents. Jacques Boissinot/The Canadian Press

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Despite these obstacles, Mr. Trudeau triumphantly achieved a majority-government victory. Almost four years later, he remains the embodiment of the party, the essence of the Liberal brand. Polls show that he is also deeply unpopular, thanks to allegations by former attorney-general Jody Wilson-Raybould that he and his advisers attempted to interfere in the prosecution of SNC-Lavalin.

The Grits are no doubt calculating that since this scandal isn’t going away any time soon, the threatened lawsuit will at least allow supporters to claim Mr. Trudeau is innocent, that he is being defamed by his opponents. In essence, the party is trying to repair the damage to Brand Trudeau, which remains the Liberal Party’s only asset. Unless it can repair that damage, it cannot win the next election.

But oh my, it’s a high-stakes gamble. In Question Period on Monday, Mr. Scheer was ruthless in attack.

“The Prime Minister has done everything he can to try do shut down criticism," he alleged, by having House justice committee hearings terminated and by throwing critics out of caucus. Let’s get the Prime Minister under oath right now, he urged, to account for his actions surrounding SNC-Lavalin. "When will they start court proceedings?” (This is not how libel trials work, but never mind.)

House Leader Bardish Chagger accused Mr. Scheer of deleting allegedly defamatory tweets and of “attending the same rally as white supremacist Faith Goldy,” a statement that Mr. Scheer called “disgusting." Mr. Scheer has maintained he was unaware that nativist protesters were part of a pro-pipeline rally he addressed in February. But labelling Mr. Scheer as soft on racists will be a core Liberal message in the months ahead.

To most observers, it was advantage Scheer.

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There could be another reason behind the threat of legal action. When Michael Wernick, Clerk of the Privy Council, first testified at the justice committee in March, he declared: “I’m deeply concerned about my country right now and its politics and where it’s headed. … I worry about the reputations of honourable people who have served their country being besmirched and dragged through the market square.”

Mr. Wernick’s indignation, his sense of grievance, might have opened a window into the thinking within Mr. Trudeau’s circle. There have been precedents. In the last years of the Harper government, Conservatives began equating their party’s interest with the national interest, which meant anything was justified – even talk of “barbaric cultural practices” – to secure victory.

The voters decided it was time for a change of government.

The Trudeau team may have convinced themselves that the Liberal Party’s interests are synonymous with the national interest. (Liberals tend to think that way at the best of times.) By such reasoning, the SNC-Lavalin affair is not just a threat to the Liberal Party’s re-election hopes, it is a threat to the country itself. Andrew Scheer is a menace. He would gut the social programs that the Liberals implemented in the national interest. He would stoke anti-immigrant sentiment. (There is no credible evidence for such a claim.) He must be kept from power at whatever cost. If that means gagging him with a libel notice, so be it.

If that is what the Liberals are thinking, then they really are in trouble.