The verse Lev melech beyad hashem refers to a step in which you have no bechira: After the nomination of the King, we need to see every action of the king as a realization of the intent of HaShem. [see Rabenu Yona on Avot 2, 3] 02
But in a previous step, before the election, this vision is erroneous. If a candidate "A" says "I will make an action "f" and you disapprove this action, you must see your duty ad a duty to act against "A". If "A" b es come president because of your lack of opposition, you may be regarded as a shutaf of the action "f", as the famous expression: פרתו של רבי אלעזר בן עזריה in Mishna betsa (2, 8) and Shabbat (5, 4).