"We'll be taking the fight to the government," he said. "They are on notice that their easy days are over."
Key said he expects Cunliffe to prove a more articulate and forceful opponent than Shearer. But he added that Cunliffe appeared already to be failing on his pledge to keep the arguments about policies and not personalities.
Polls indicate that Key's center-right National Party remains the most popular single party, but that it doesn't have as much support as the Labour and Green parties combined, which could form a left-leaning coalition.
"I treat any leader of the opposition with respect, and if you don't you're pretty silly as a prime minister and it comes at your own peril," Key said Monday. "And this is a really tight election, we know that."
Cunliffe is a former diplomat and health minister who has a master's degree from Harvard Business School.