Thomas Mulcair's leadership bid has received a double boost, with NDP MP Jack Harris throwing his support behind the Quebec MP and with rival candidate Martin Singh saying he's his No. 2 choice on the ballot.

Harris joined a lengthy list of caucus endorsements for Mulcair, but the supporters will no longer include Sana Hassainia, who announced Wednesday she was switching her allegiance from Mulcair to Brian Topp.

She said after watching the debates she's now decided that Topp, who does not hold a seat in the House of Commons, is the best person to beat Prime Minister Stephen Harper and lead a progressive government.

Harris, in contrast, said Mulcair is the candidate who "stands out as the one who has the experience and skills necessary to defeat Stephen Harper in the next federal election and help us form the first NDP government."

The MP from Newfoundland and Labrador is well-known in the caucus and Harris is currently the party's justice critic.

Singh, meanwhile, made a point of endorsing Mulcair as his backup pick during an interview Wednesday on The Current with host Anna Maria Tremonti and said he is encouraging his supporters to pick Mulcair as their second choice.

Singh said he had made a decision "to mark Thomas Mulcair as my second choice and we've started communicating that through our campaign team to our supporters."

Advance voting is underway and members are using preferential ballots — ranking the seven candidates in order of preference. A member's first choice is applied until that person drops off the ballot on the day of the convention in Toronto on March 24, then his or her second choice is counted, and subsequent choices if necessary.

Only the advance ballots are preferential, and because most NDP members are expected to vote in advance, the ranking of candidates is strategically important for the campaigns.

Singh says he's 'running to win'

Singh, a pharmacist from Nova Scotia, made the comments amid speculation that his campaign and Mulcair's are working together.

"There's been some talk in the media, and Brian Topp has accused me of being in cahoots with Tom, and that is entirely incorrect," Singh said.

He said the suggestion was insulting and demeaning to his supporters.

"I am running to win and in order to win I have to beat everybody, Tom included," he said.

Singh, left, looks at Mulcair during a debate in Quebec City in February. Niki Ashton is on the right. (Clement Allard/Canadian Press)

Singh said there is "no formal arrangement" between the two campaigns and that he is making his second choice known because he felt it was his responsibility to share it with his supporters.

"I felt that it was important that, how shall I put it, if it cannot be me then I wanted to make sure that it's someone who I felt had policy ideas that were very close to my own," he said.

Allegations of complicity between the Singh and Mulcair camps surfaced after the Montreal debate on Sunday, when Singh attacked Topp and Peggy Nash, two of Mulcair's top rivals.

Mulcair has categorically denied any co-ordination with Singh.

The Canadian Press reported earlier in the week that a former campaign worker for Singh, Sukh Johal, is now working on behalf of Mulcair.

Johal said he signed up thousands of new NDP members in British Columbia on behalf of Singh and he's urging those people to support Mulcair as their second choice.

Johal said he had suspected Singh would urge his supporters to rank Mulcair as their number 2 choice sometime this week.

Singh's campaign has described Johal as a disgruntled ex-member of their team who was let go for "non-performance and non-compliance." Singh told Tremonti that it's difficult for him to put any truth in what Johal has been saying about the campaigns and that he hasn't spoken to him "in quite some time."

Following Sunday's debate, several of the candidates were asked about their second choice, and only Topp named one, citing Manitoba MP Niki Ashton as the fellow candidate who impressed him.

Nathan Cullen and Paul Dewar are the other candidates vying for the NDP leadership job. Cullen also won a caucus endorsement Wednesday from northern Ontario MP Bruce Hyer.

Dewar, an Ottawa MP, reacted to Singh's encouragement for his supporters to pick Mulcair as their number 2 choice. He said any notion that Singh's supporters will follow his lead is "simply incorrect" and that Dewar is receiving strong support for members' first and second choices.

The NDP leadership race was prompted by Jack Layton's death last summer and it has lasted almost seven months.

About 131,000 NDP members are eligible to vote for the new leader.