Education Minister Simon Birmingham is digging in against Catholic educators and will not apologise for likening their Victorian branch to Judas for backing Labor in the Batman byelection.

Senator Birmingham is under fire from the opposition - and from some anonymous internal critics - for suggesting Catholic Education Melbourne had been "bought by a few pieces of silver" from Labor.

In the biblical story, Judas betrays his friend Jesus for 30 silver coins. In this case, the "silver" refers to Labor's promise to restore $250 million in funding for Catholic schools in its first two years of government, if elected.

Education Minister Simon Birmingham. Credit:Dominic Lorrimer

That pledge was attacked by the public school parents' lobby, which dubbed it an "irrational and illogical" special deal in a scathing dispatch reported by Fairfax Media on Tuesday.