12/18/2014 06:21 EST | Updated 02/17/2015 05:59 EST

Two senior Manitoba New Democrats back challenger to Premier Greg Selinger

WINNIPEG - A leadership bid by one of the five senior cabinet ministers who tried to get Manitoba Premier Greg Selinger to quit appeared to be all but official Thursday.

Two longtime NDP advisers said they were backing Theresa Oswald's yet-to-be-confirmed campaign to lead the provincial New Democrats. One of them said he had signed nomination papers for her.

"I talked to Theresa a few weeks ago and urged her to run ... and as it's getting closer to decision time, I was approached to sign her papers, which I did proudly," said Eugene Kostyra, a one-time cabinet minister and senior adviser to former premier Gary Doer. Kostyra retired in 2007 but was a campaign adviser to Selinger in the 2011 provincial election.

"It's time to move on and I think Theresa is the one that can take us back into contention."

Also leaving the Selinger camp is Anna Rothney, who told the Winnipeg Free Press she is taking a leave of absence from her role as a top Selinger adviser, in part to help Oswald's campaign.

Oswald had not officially launched her leadership bid as of Thursday and did not return messages. She said earlier in the week she was still gauging support from party members but was close to making a decision.

Selinger was essentially forced into a leadership contest after Oswald and four other ministers suggested he consider resigning to help the party win the next election, slated for April 2016.

Selinger stirred up voter anger last year by raising the provincial sales tax, and the party has lagged behind the Opposition Progressive Conservatives in opinion polls ever since.

Oswald and the other rebel cabinet ministers quit their portfolios to sit on the backbenches. Selinger said he was staying on as premier and challenged his critics to run against him in a leadership contest at the NDP's annual convention March 8.

The party has set Jan. 6 as the deadline for candidates to enter the race. The same day is the deadline for candidates to sign up new members who will choose delegates to the convention.

One political analyst said Oswald is showing surprising strength by securing big-name backers within the party.

"I think what it indicates is that people in the party are not as intimidated by the way that (Selinger) has positioned himself in this leadership race by staying on as premier as we might have thought," said Royce Koop, who teaches political science at the University of Manitoba.

"We have big names, historically and in a contemporary sense, that aren't afraid to back (Oswald)."

Oswald served as minister of health and, more recently, minister of jobs and the economy. She has held the Seine River constituency — a former Tory suburban stronghold — for the NDP since 2003.

Koop suggested Oswald has an uphill battle against Selinger, despite the backing of high-profile party members. Some party members are upset that Oswald and the other rebels went public with their criticism, he said. Selinger also appears to have solid backing from the labour movement, he added.