The Labour Party is accusing National of waging a dirty tricks campaign against one of its contenders for the Mount Albert by-election, even before he has been selected.
It says National has been trawling through papers written by Labour's potential candidate, David Shearer, in an attempt to discredit him.
Labour claims National has fed information to political bloggers about comments Mr Shearer has written about private security contractors.
In the past, Mr Shearer has suggested that private contractors be used in war-torn countries where no better alternative exists.
National has denied feeding information to bloggers.
A spokesperson for Prime Minister John Key says the accusations are unbelievable coming from Labour, the master of dirty tricks' campaigns.