When Patricia Fields, the owner of a trendy downtown boutique of the same name (10 East Eighth St.), sent her first advertisement copy to Vogue magazine, she had no idea it would be rejected. It was, after all, the same format she has run in local publications. The problem, it seems, was two words in the copy: "DUMP REAGAN."

"They sent it back, asking for nonpolitical copy," Fields said the other day. "I said . . . I am not going to make a new ad campaign to please them. My whole store has been turned over to this. I have asked artists to design posters in this theme. I am registering people to vote. I have red, white and blue banners -- it looks like the convention hall in here."

Vogue, which would have published the ad in this month's issue on a page devoted to small New York boutiques, claimed to know nothing of the rejection. "She probably had some problem with credit," a spokesman for the magazine stated.

"Nonsense," retorted Fields. "The page was a cut-rate page they had been pressuring me for months to go onto. When Reagan was elected the first time I was numb. This time I am ready for action."