There isn't a direct verb form of "emphatic". However, you can use verbs to express the meaning of being emphatic:
* Assert
* Declare
* Insist
* Stress
* Emphasize
* State
* Affirm
* Proclaim
* Underscore
For example, instead of saying "He was emphatic about his opinion," you could say:
* "He asserted his opinion strongly."
* "He declared his opinion with force."
* "He emphasized his opinion."