Can "decided" be used this way?

Is it possible to use the verb "decide" as follows? I've seen some cases recently, for example:

"I'm decided I'm voting Republican".

I've never seen it used this way before. Is it grammatically correct?

"I'm decided I'm voting Republican" is a version of "I have decided I'm voting Republican. It is a correct usage of the word. Perhaps not very common, maybe even a regionalism, but still within the standard.

"We are agreed"and "we have agreed" are similar structures. It goes back to the "competition" between the verbs "to be" and "to have" as the auxiliaries of choice in the English language. In some cases, this struggle remains undecided. So, language users are free to choose from co-existing standards.

BTW, "decided" in your example is in the past participle, "am", of course is present tense. Here it is used to render the statement passive in the simple present tense. As in, for example, "He is taken ill." Contrast this with, "He has taken ill." (The present perfect simple.)

• I'm decided I'm voting Republican

is not an example of the verb decide in present tense.

Rather, it's an example of the present tense of the predicate adjective (be) decided, which can take a that complement, just like decide, the verb it's derived from.

This is in distinction to I have decided that S. That's the present tense of have decided, which is the Perfect construction of the verb decide.