Rival camps in the NDP leadership contest certainly think so, although Mulcair's adamantly denies it.
For weeks there's been rampant speculation that Singh, a Nova Scotia pharmacist with little chance of winning, had allied himself with Mulcair and would eventually throw his support to the acknowledged front-runner.
But that speculation has hardened into virtual certainty for many New Democrats after Singh's performance in the last two leadership debates, in which he launched the harshest attacks yet seen in the contest against two of Mulcair's top rivals — Brian Topp and Peggy Nash.
Indeed, Singh's attacks were so fierce that the party's chief electoral officer has issued a memo to all campaign teams warning that any candidate who uses unparliamentary language at future party-sponsored events will be cut off, a financial penalty imposed and a public apology required.
"It doesn't take a rocket scientist to put two and two together and conclude that Mr. Singh's continuing over-the-top and malicious attacks on Brian are part of a co-ordinated strategy, " says Topp campaign spokesman Jim Rutkowski.
"But you would have to ask Mr. Mulcair if he, or his campaign, is involved in any way."
The Singh camp did not respond to repeated requests for comment over several days.
However, Mulcair's campaign director denies Singh is acting on the Montreal MP's behalf.
"The idea that we are somehow co-ordinating debate performances or using Mr. Singh as an 'attack dog' is entirely fabricated and, if I may say so, somewhat condescending to Mr. Singh," says Raoul Gebert.
Privately, all five rival camps aren't buying it. They have little doubt that Singh is "doing Tom's dirty work," as one senior New Democrat put it, allowing Mulcair himself to remain resolutely above the fray.
"Basically, he's doing Tom's bidding for him," asserts a top strategist for one rival camp. "Tom has told him, 'I'm the positive guy, I need you to be my attack dog.'"
Rival camps noticed early on that Mulcair and Singh seemed to hang out together during all-candidates' events. Two camps say they've actually heard from some New Democrats who've reported that Singh openly told them he intended to back the Montreal MP.
"It's been known on the inside for months that they're in cahoots," agrees an insider with a third camp.
But Gebert says the fact that Mulcair and Singh get along is proof only of the fact that Mulcair has cultivated good relations with rival candidates — smart strategy in a race where party members' second choices will likely determine the outcome.
"We are proud to have retained positive relationships with all of the other leadership camps by running a resolutely positive campaign," he says.
"Thus, to allude to some 'chummy' situations at different moments during the campaign simply proves that other candidates are indeed looking to Tom as a second choice and we are proud to have received the endorsements of former leadership candidates Romeo Saganash and Robert Chisholm."
Initially, the other camps assumed Singh would drop out of the race early and side with Mulcair. But after the most recent leadership debates they're convinced he'll hang on until the March 24 convention, playing hit man for Mulcair in the meantime.
At the Winnipeg debate two weeks ago, Singh repeatedly went after former party president Topp over his proposal to eliminate the tax exemption on most capital gains. Singh maintained the proposal would be devastating for charities, including a sexual health organization with which he's involved.
"I am terrified of the impact your plan would have on our work protecting a woman's right to choose," he said.
Topp replied that his proposal could be tweaked to protect charities but Singh wasn't satisfied, at one point urging Nash to join in his condemnation of Topp. She declined.
Last weekend in Montreal, Singh returned to the subject, repeatedly accusing Topp of lying about the impact of his capital gains proposal.
"Why did you lie the first time and switch your story? Why did you lie?" he demanded.
A bemused Topp chided Singh for adopting a tone that was "a bit harsh for a debate among colleagues."
But Singh wasn't done. In his closing remarks, he used his precious 90 seconds of unfiltered self-promotion time to launch a broadside against Toronto MP Nash, asking; "How can someone who's never lived in Quebec think she understands Quebec?"
Of the seven contenders, only Mulcair and Topp have lived in the province. But lest anyone think he was suggesting the Quebec-born Topp might be a viable choice, Singh then returned to the capital gains issue, maintaining that Topp had accused everyone who disagrees with him of not being a good New Democrat.
Noting that the party's late revered leader had never made such a proposal, Singh concluded: "I think Mr. Topp would say maybe that Jack Layton was not a good New Democrat."
Singh was the only one of the seven leadership contenders who did not take questions from reporters following the Montreal debate. His camp has subsequently ignored repeated requests for comment.
In the aftermath of the Montreal debate, Eric Hebert-Daly, the NDP's chief electoral officer, sent a memo to all campaign teams noting that "candidates have at times used unparliamentary language" during recent party-sponsored events.
He said such language isn't permitted in the House of Commons and won't be tolerated in the leadership contest either.
"If during a debate or a speech that is hosted by the party, a candidate says something that is unacceptable or a personal attack, I will give the moderator or stage manager the right to cut the microphone of the guilty party," Hebert-Daly says in the memo, obtained by The Canadian Press.
"I will follow up with a full discussion and a financial penalty to the candidate. A public apology will need to be posted in the public domain as part of the penalty."
Quebec MP Francoise Boivin, who has endorsed Topp, says she was a "bit surprised and taken aback" by Singh's attack on Topp in Montreal. She felt she'd been transported for a moment to question period in the House of Commons.
"I expect that from the Conservatives, not from someone from my own party," she says.
Still, Boivin, who has indicated Mulcair would be her likely second choice, refuses to speculate on whether Singh is acting on Mulcair's behalf.
"I have absolutely no intention of playing that game."
Singh has been largely regarded as a bit player in the leadership contest. But he has recruited several thousand new party members, primarily among the Sikh community. Because his supporters are largely from a cohesive cultural group, rival camps suspect they may be more prone than most to follow their candidate should he throw his support to Mulcair.
If the outcome is close, Singh could go from attack dog to king maker.
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