Is an oath considered void if a portion of the group does not abide by the voting, after swearing on the Quran to vote for a specific person in the elections?

It is commendable to fulfill a Muslim's oath if it is lawful and there is no harm in it. If the oath is not fulfilled, the one who swore it must offer expiation. The one who swore the oath breaks it when it is not fulfilled, unless he thought that the one he swore about would obey him, but then that person disobeyed him. In this case, no expiation is required of him.