In the sentence "I do like mint ice cream" 'do' is an auxilliary verb. However, if you were responding with a "I do" in a wedding vows context, is 'do' auxilliary? It would be if you continued the line and said "I do promise", but what about when you don't?
When saying 'I do promise', for example, the main verb is -to promise, making the verb -to do an auxiliary, but if just saying 'I do', the only verb used is -to do, being the main verb in the sentence, not having what to assist.
Usually, when 'do' is applied before a verb - as an auxiliary - the strength is highlighted. In the same case, it is possible to use 'I promise', as well as using 'I do promise', what gives the state more confidence.