He was concerned about the "degradation of the public's confidence in the democratic process by Judith Collins, Cameron Slater, Jason Ede and other rotten Shylocks".
Mr Cunliffe said Mr Gibson could not pulled as a candidate.
Asked if people in Rangitata should vote for Mr Gibson, Mr Cunliffe said: "We are campaigning hard for the party vote....People will make up their own mind."
With two weeks to go the election, Mr Cunliffe is campaigning on in Hokitika and Greymouth with local MP Damien O'Connor.
He is due to meet the Pike River families later today - the families of 29 men killed in the Pike River mine explosion in November 2010.
"We will be reinforcing to them our commitment that we will be doing everything possible to retrieve the bodies from the Pike River mine."