When speakers are trying to impress audiences with their rhetoric, they often seem to feel that the extra syllable in “importantly” lends weight to their remarks: “and more importantly, I have an abiding love for the American people.” However, these pompous speakers are wrong. It is rarely correct to use this form of the phrase because it is seldom adverbial in intention. Say “more important” instead. The same applies to “most importantly”; it should be “most important.”