Once again the prime minister and the opposition leader were in each other sights, with NDP Leader Tom Mulcair attacking Stephen Harper with questions about the investigation into the Guelph, Ont., robocall scandal, and at one point accusing Harper of using "weasel words."

It was the first time both leaders had faced each other in the House of Commons for question period since Parliament took a break before Remembrance Day.

Again and again, Mulcair asked Harper if Conservative staffers were ordered to provide specific testimony about Michael Sona, the former Conservative campaign worker facing criminal charges in the Guelph robocalls affair.

Sona is charged with trying to prevent a voter from casting a ballot in incidents where thousands of voters in the federal riding of Guelph received robocalls directing them to the wrong polling stations on election day in 2011.

Mulcair said Sona was actually on holiday in Aruba, "on a beach in flip-flops" when several Conservative staffers told Elections Canada investigator Al Mathews the former campaign worker confessed to them about being the mastermind behind the robocalls.

Sona denies the charges and has said he believes he's been set up to take the blame.

Harper, as he usually does, told Mulcair that he believes anyone responsible for the robocalls must be held accountable.

Mulcair, asking all the NDP questions for the first half of question period, switched from the robocalls scandal to the Senate expenses scandal.

He inquired if the Prime Minister's Office is being investigated by the RCMP over the writing of a $90,000 cheque to Senator Mike Duffy by the prime minister's former chief of staff, Nigel Wright. Harper answered the PMO is not being investigated but is "working with" the RCMP and providing it with information.

'Weasel words,' again

Mulcair shot back, "Now there's a classic, to my knowledge. That's a new one, in the top 10 .... of weasel words,' uttering the last two words almost under this breath, and quickly moving on to his next question.

The House erupted, with cabinet ministers Tony Clement and John Baird begging Speaker Andrew Scheer to throw Mulcair out for using unparliamentary language.

Scheer, smiling, warned the Opposition Leader not to make use of "animal references", pointing out he'd admonished Mulcair once before about mentioning rodents. A few weeks ago, in a question to Harper, Mulcair pressed, "No weasel words, [answer] yes or no."

Mulcair, laughing, said, "I'm such puppy dog on this issue," perhaps referring to commentators who've likened him to a pitbull.

Earlier, Liberal Leader Justin Trudeau, when Harper asked him to come clean about the Liberals' missing "$40 million", a reference to missing money in the Chrétien-era sponsorship scandal, Trudeau retorted, "I'm not taking lessons in accountability from someone whose fishing buddy is Rob Ford."

Both opposition leaders brought up the name of Jenni Byrne, Harper's deputy chief of staff. Trudeau suggested Byrne advised Conservative staffers to delay talking to Elections Canada investigators and Mulcair suggested she also helped cover up the Senate scandal.

Harper continued to insist his office is fully co-operating with both Elections Canada and the RCMP, and twice levelled a charge against Mulcair that for once did not act as a catalyst for much heckling: "When you throw mud, you lose ground."