Prime Minister Helen Clark says the Labour Party is very unlikely to support any moves to hold a binding referendum on MMP.
The National Party says it will give people the chance to vote on the electoral system, if it becomes government after the election.
Miss Clark says a referendum is not Labour policy, and National does not like the thought of having to share power with other political parties.
However, she says power-sharing is not a bad thing, even if it means working hard to mobilise majority support.
National Party leader John Key says the party would hold a binding referendum on MMP no later than 2011.
He says voters have been waiting for some time to be able to vote on the electoral system, since the first MMP election in 1996.
Mr Key says if the majority of voters decided MMP was not their preferred electoral system, National would offer a range of electoral systems to replace it.
He says the first referendum would simply ask people whether they support MMP.
Mr Key says he believes most people would want to dump MMP, but to replace it with another system of proportional representation.