Entrenching means changing electoral law so a 75 percent majority vote in Parliament would be needed to abolish the seats, which is a bottom line for the Maori Party in any post-election negotiations.
Getting rid of the seats is a National policy.
"My preference is for everyone to go on the universal franchise," Mr Key said on TVNZ's Breakfast programme.
"If you look at entrenchment ... that is diametrically opposite to that position.
"Now we don't know what any of those negotiations would look like, but it would be a very large step."
Labour has already said it would be prepared to back a Maori Party bill entrenching the seats, but Mr Key reiterated he would not rule such a move out, or in, ahead of the election.
Miss Clark said Mr Key had to admit entrenchment was out of his step with his party's wishes.
"I think when cornered he had to admit that entrenchment was the opposite of abolition. Their policy is abolition," she told reporters this afternoon.
"Under the National Party policy Maori people would only have one more election for the Maori seats and that would be 2011 and then they would be gone by that lunchtime.
"So he's had to admit that in essence entrenchment doesn't square with that."
Miss Clark said the turf war between the Maori Party and Labour in the Maori seats would not affect post-election talks.
"It's a two horse race but I think each of us recognises in the other that we have an interest in the advancement of Maoridom. And so that probably gives us a lot to talk about rather than less to talk about."
She was confident Labour could retain the three Maori seats it holds.
Miss Clark was asked about reports that the Maori Party saw itself as a Treaty partner -- thereby claiming to represent all Maori.
Maori Party co-leader Tariana Turia yesterday clarified the party was looking for a Treaty relationship.
"I've been following some of the debate about that. I don't think it's as clear as the party claiming Treaty partner status but obviously what they are interested in is a relationship which draws on the principles of the Treaty and I completely understand that," Miss Clark said.
