Originally Posted by FaceLeg

If you mean that one is speaking in defense of liberty, then perhaps:

"Defensio libertati"

Which is literally "Defence of liberty"

To the best of my knowledge the following would mean "In defence of liberty"

"In defensione libertati"

This "Defensio, Defensionis" means: defense/protection; act of defending; argument/justification in defense, excuse legal maintenance of a right; legal prosecution, punishment;

Bold for the meaning one imagines you are aiming for.

By Latin is not great ( < 1st year), but after consulting my three paper dictionaries, and one digital one, the above is my best try.