There are several synonyms for the phrase "lose power". One such phrase is "lose control", which implies that the subject has lost their ability to command or manage a situation. Another common synonym is "weaken", which suggests that something has become less effective or potent. "Diminish" is another term that can be used, meaning to decrease or become less. "Decline" is another good option in this context, indicating that something has gradually deteriorated or lost strength. Finally, "falter" implies that something is struggling or losing momentum, while "deplete" suggests that something has been drained or exhausted.