K.P.
My first guess--he has found something more interesting to him he would rather be doing!! Also--are you giving him a warning like "bath in 2 minutes", or just making him stop playing immediately? Sometimes kids get upset if you don't give them that warning time, and he has reached that age where his play is becoming more elaborate.
Secondly--same as the other moms. He really doesn't need a bath every night. Even your pediatrician would confirm this. Every other night would be sufficient, or 3 nights a week, whatever your schedule fits best. The best way to approach this so that it doesn't become a "i win you lose" situation, is to just drop it out of the routine one night without even mentioning it. Just skip right over it one night. If he mentions it just simply state "Mommy decided that a bath every other night might be better from now on." Then ask him "what would you like to do with that extra time form now on?" That might give you a clue as to why hes been fighting you on the bath.
However--the days that it is bath night, he doesn't have the right refuse, but you really don't want to make the bath a power struggle. So something like a new bath toy would likely do the trick. If you have never tried those tub color dyes--my son loved those when he was 2. Little tablets you drop in that change the water colors. You can drop in blue and red to get purple and so on. Maybe make something like that a once a week treat if he doesn't fight you the other nights.