LONDON — The motion before the House of Commons, the mother of all parliaments, was among the gravest that any democratic body can consider: Should Britain launch airstrikes on Syria?

But instead of a substantive weighting of the merits of war, the debate that unfolded Wednesday at the Palace of Westminster turned into a brawl over a peculiarly post-9/11 schoolyard taunt.

On Tuesday evening, British Prime Minister David Cameron had urged members of his Conservative Party to support his push to drop bombs on the Islamic State militant group in Syria — and to resist the calls of those seeking restraint.

“You should not be walking through the lobbies with Jeremy Corbyn and a bunch of terrorist sympathizers,” the prime minister said, referring to the leader of the opposition Labour Party.

The comment was made behind closed doors. But it wasn’t long before it was leaked to virtually every news outlet in Britain.

By the time Cameron and Corbyn squared off Wednesday morning from the green benches of Parliament — appropriately separated by two swords’ lengths — “terrorist sympathizers” had become the phrase on every British politician’s lips.

Cameron had barely begun to make his case for airstrikes when former Scottish National Party leader Alex Salmond interrupted: Would the prime minister apologize for his “deeply insulting remarks”?

No, he would not.

But, Cameron hastened to add, he respected his opponents and “there is honor in any vote.”

Then Emily Thornberry, a Labour politician, jumped in: Amid all the debate over what to call the Islamic State, would Cameron renounce his decision to affix a “dangerous and untrue” label to his political opponents?

“I think that it is time to move on,” the prime minister said gruffly.

Then Corbyn himself demanded an apology, saying Cameron’s comment “demeans the office of the prime minister.”

Cameron stayed seated, and quiet.

Amid all the talk of “terrorist sympathizers,” the question of going to war in Syria got somewhat lost.

The prime minister’s remarks have a context.

Corbyn, who proudly calls himself a radical leftist and who won the party leadership in September on an unapologetically dovish platform, has expressed support for the anti-American former Venezuelan leader Hugo Chávez.

One of Corbyn’s top aides, John McDonnell, has said that the IRA should be “honored” for bombings that brought “Britain to the negotiating table” in Northern Ireland.

Another top Corbyn adviser, former London mayor Ken Livingstone, said last week that former prime minister Tony Blair was responsible for the bombings of London trains and buses in 2005 because he had led Britain into war in Iraq.

Cameron and his allies have used those comments, and others, to suggest that a Prime Minister Corbyn would be dangerous for British national security. Earlier this fall, Cameron even went so far as to say that Corbyn “hates” Britain.

But analysts said his comments on Tuesday crossed a line and are likely to hurt his case for war — especially because he may need Labour support to win Wednesday night’s vote. The Spectator’s Sebastian Payne wrote that Cameron “will be kicking himself this morning.”

On Twitter, the mocking comments came pouring in, as people fessed up to the prime minister’s charge:

Karla Adam contributed to this report.

Read more: