In a democracy those who support a candidate or party have the duty of voting that party into power. However they have an even greater responsibility if the party wins which is to hold the government accountable and become the biggest pressure group for constructive criticism.
The reason this constructive criticism and not blind support is so important is, those in power will have a greater tendency to listen to those whom they feel are their constituency and base. They may see the criticism of opposition as biased and just the cry of losers.