Shortly after question period, Dewar complained to Speaker Andrew Scheer that Harper had falsely accused him of opposing a trade deal with Jordan.
Scheer waved off the complaint and suggested Dewar continue his debate with Harper elsewhere.
So he did — right there in the centre aisle of the Commons, which is meant to prevent unseemly confrontations by keeping government and opposition MPs at a safe distance.
Instead, in full view of parliamentary video cameras, the pair shook hands before engaging in a brief but animated discussion.
Dewar says he told the prime minister he was mistaken on the Jordan trade deal; Harper, he added, promised to review Dewar's record on the deal.
Harper had earlier been forced to retract his accusation that Dewar wanted Canada to pull out of trade talks with the European Union.
"It's not a good day for the prime minister, I guess," Dewar said later.
The last time an MP crossed the floor of the Commons, it ended in an ugly verbal dust-up that nearly escalated into a brawl.
Shortly before Christmas, an angry Government House leader Peter Van Loan stormed across the aisle to confront his NDP counterpart, Nathan Cullen.
Microphones were shut off but New Democrats later accused Van Loan of using profanity, prompting NDP Leader Tom Mulcair — Cullen's seat mate — to warn him against threatening his House leader.
New Democrat MPs, including Dewar, surrounded Van Loan and urged him to leave. Eventually, Defence Minister Peter MacKay ushered Van Loan back to the government benches.
Conservatives claimed it was Mulcair who lost his temper but Van Loan subsequently apologized for using "an inappropriate word" during the contretemps.
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