Former Labour MP for Blaenau Gwent Llew Smith says independent challenger Peter Law will win his old seat, overturning a 19,000 Labour majority.
Llew Smith says local people are angry with Labour
In a BBC Wales interview Mr Smith did not voice personal support for Mr Law but said he suspected he would win.
Mr Law, Assembly Member for Blaenau Gwent, left Labour in a row over all-women shortlists.
But Labour candidate Maggie Jones expressed doubt that he could fight Blaenau Gwent's corner at Westminster.
Llew Smith, an MP for 13 years before standing down at this election, said local people resented the way the Labour candidate Maggie Jones was selected through an all-women shortlist.
Peter Law is recovering from surgery for a brain tumour
"That (Peter Law winning) may seem hard to believe as it's one of the safest Labour seats in the country," he said.
"But it's a reflection of the anger felt by local people because local people are saying to me that they're (the Labour party) treating us as idiots.
"Here are local people who had the good sense to elect the two giants of the 20th Century Labour movement in Michael Foot and Nye Bevan. But they're basically saying to those people now 'Ah, but you're not intelligent enough to decide on Llew's successor'".
Earlier this week, Labour's former Home Secretary David Blunkett visited the constituency in support of Maggie Jones, the Labour candidate, and urged Mr Law to stand down from the race.
But Mr Law, who deserted the Labour party to fight the seat as an independent, announcing his candidature just a few days after a six hour operation for a brain tumour, said there was a genuine issue to be decided by the electorate at this poll.
"The fact is that people do understand the argument that we are being used by Tony Blair and Peter Hain to give somebody a job for life," he said.
"We only ever wanted the right of choice. We only wanted an open shortlist to pick, on merit, the right person for the job and to do that with transparency.
"It's been a disgrace the way people have been taken for granted here and I believe that's the reason they are reacting in the way they are.
Labour candidate Maggie Jones has robustly defended her party
Labour candidate for Blaenau Gwent, Maggie Jones, who was selected from an all-women shortlist, said: "What (the voters) really want is a Labour MP, a Labour representative in Westminster, who can take up (their) issue.
"If Labour are successful, if we do win the election, what they will want is somebody who can talk to the minister, talk to the Cabinet and actually speak up and fight the corner for Blaenau Gwent.
"I'm not sure that an independent MP can do that."
Other candidates in Blaenau Gwent conceded that Peter Law was popular with the voters.
Brian Thomas, the Liberal Democrat candidate, said he thought the independent was the "bookies choice"
But he said: "We have got a strong Liberal vote out there so hopefully they will remain in the ranks."
Conservative candidate Dr Phillip Lee said Mr Law had "a lot of support" and was " a very popular person on the ground".
"However, I'm all about trying to increase the Conservative vote and I think the core Conservative vote is never going to vote Peter because they remember he is a Labour member of the assembly."
Plaid Cymru's candidate John Price said: "The only way that people of Blaenau Gwent can give the party (Labour) a bloody nose is by voting Plaid Cymru, that's where the true protest vote within Blaenau Gwent really lies."
Peter Osborne is the UK Independence Party candidate in Blaenau Gwent.