The pair yesterday confirmed their intention to secure the party's leadership, which is set to be decided by a party-wide vote most likely to take place in December.
It follows Cunliffe's announcement yesterday that he would formally resign as leader on Tuesday, a decision which will trigger Labour's leadership primary run-off.
But in a decision that will further confound his critics, he will enter that contest in a bid to win his job back.
Soon after Cunliffe's announcement, Robertson also confirmed he will enter - and said the election result showed it was time for a new generation of leadership and change for Labour.
It is also possible the field will get bigger; David Shearer and Andrew Little are yet to rule out having a go themselves. Not all of Cunliffe's critics in caucus are fans of Robertson and some are pondering alternatives.
The contest is unlikely to happen until the end of the year, with Labour's Council aiming to have a review of the election completed by the start of December and many in caucus want the review completed before the leadership is decided.
Despite the blood-letting in Caucus last week, Cunliffe said he was confident he would be able to reassert control over the MPs if he was re-elected leader.
He said some of their comments over the past week had been hard to take and "ill-judged".
But he said they were were professional and capable of putting the party's interests first.
However, he is already facing criticism for not putting the party's interests first himself by not standing down fully.
Yesterday, one MP said his "non-decision" was disappointing and had damaged the party when it needed a clean break.
"It just plays into the well-accepted view that we are a party that can't run ourselves. Now that's going to be the perception for a while to come."
Cunliffe was confident he still had strong support across the members and unions that handed him the job last year.
Robertson said this contest was a completely different ball game - despite having the same contestants.
He did not believe Cunliffe's prior resounding victory with the members and unionists would be repeated this time given the election results.
"This starts from scratch. A lot has happened in the last year and I'm confident I can get support across all three groups."
If Little opts against standing or is tipped out of Parliament on the final results of the election, his support could prove critical for one of the others because of his ability to deliver the union vote.
He is likely to side with Robertson if there are no other contenders.
One of Cunliffe's 2013 supporters, Iain Lees Galloway, would not comment when asked if he would support Cunliffe again, but said he was pleased there would be a contest for the role.
"I think the membership will appreciate being given the opportunity to have a say."
After Cunliffe's announcement yesterday, Labour Party president Moira Coatsworth said the party would decide by Thursday when that contest would be.
It will not formally be triggered until Cunliffe officially resigns at caucus on Tuesday.
