The National Party believes more senior Cabinet ministers could resign in the lead-up to next year's election if Labour keeps struggling in the polls.
Education Minister Steve Maharey yesterday announced he would retire from politics in the second half of next year to become Massey University's vice chancellor.
The move paves the way for Prime Minister Helen Clark to promote several fresh faces to Cabinet in a major reshuffle expected ahead of Labour's annual conference in a fortnight.
Mr Maharey yesterday said the role at Massey, where he has studied and taught, was a "dream job" and his retirement was not a reflection on Labour's prospects at next year's election.
However, Labour has been trailing National by about 10 percentage points in most recent political polls.
National's deputy leader Gerry Brownlee yesterday said the position Labour was in could have a corrosive effect.
"We wish him well in whatever he does, but suspect he won't be the last of the front bench to announce their intention to retire."
Mr Maharey is the fourth ranked minister in Cabinet, holds the education and social development portfolios as well as a range of smaller ones, and has been a key campaign strategist in each of the past three elections.
His departure opens up a Cabinet spot and also provides a vacant seat on the Government's front bench.
Another Cabinet spot is that left by disgraced MP David Benson-Pope.
Possible contenders for Cabinet elevation include list MPs Shane Jones and Maryan Street and Pacific Island MP and Voluntary Sector Minister outside Cabinet Winnie Laban.
Candidates for promotion inside Cabinet include Immigration and Communications Minister David Cunliffe, Energy and Climate Change Minister David Parker, Building Issues Minister Clayton Cosgrove, Labour Minister Ruth Dyson and Conservation and Housing Minister Chris Carter.
As well as the education and social development portfolios, Mr Benson-Pope's environment portfolio will also be up for grabs.
There might also be another ministerial spot available outside Cabinet with Dover Samuels, who has said he will retire at the next election, due to stand down in the near future.
Mr Maharey is the third Labour MP to announce retirement in recent days. Earlier this week list MPs Dianne Yates and Ann Hartley said they would retire after the summer recess.
Mr Maharey revealed yesterday the death of his wife, Liz, from cancer in 2004 had been instrumental in him looking to a life outside politics and he had contemplated stepping down at the 2005 election.
"Politics was a project that I began with Liz back in the 1981 Springbok Tour," he said.
"When she died you thought to yourself 'I've got to rethink what I'm doing here with the rest of my life'. In that sense it caused a period of thought that's gone on ever since."
An emotional Mr Maharey said he contemplated quitting ahead of the 2005 election, but Miss Clark was "enormously supportive at the time" and gave him time off to take a break in Britain and ultimately convinced him to stay.
He said he had not applied for the vice chancellor's job, but had expressed an interest when he was shoulder-tapped and then shortlisted by Massey.
Mr Maharey said he believed he was leaving a vibrant Labour Party that had sufficient energy and ideas to win the next election.
Miss Clark said Mr Maharey had made a tremendous contribution to the parliamentary Labour Party and she wished him well for his life beyond Parliament.
Massey University Council today said it was "delighted" he had accepted the role.
The Association of University Staff also welcomed Mr Maharey's appointment.