There are several synonyms for the phrase "become weaker." One common word is "decline," which means to gradually lose strength or power. Another synonym is "deteriorate," which suggests a more rapid or sudden decline. "Diminish" is another option, describing a decrease in intensity or potency. "Weaken" is a straightforward synonym, indicating a loss of strength or effectiveness. Finally, "falter" implies a wavering or uncertain quality, suggesting a weakening of resolve or determination. All of these words can be used to describe a variety of situations in which something loses its power or influence.