For Mordecai the Jew was second in rank to King Ahasuerus, and he was great among the Jews and popular with the multitude of his brothers, for he sought the welfare of his people and spoke peace to all his people.
This is the last sentence in the book of Esther. Mordecai became a wildly successful political leader because he visibly lived out the outcomes of the Law. God wanted the Jews to take care of their poor and their needy. A major part of the political instruction given in the Old Testament has to do with caring for the very poorest in their society. This exemplified Mordecai's political work among the Jews in Persia. He worked to save them all at their poorest and most helpless. And God blessed his commitments.
The simple reason that Mordecai was loved by the people was that he really cared for them. They were prosperous and ruled well which led to peace and real love for their leaders. God blessed the rich dedication of that one man had to biblical principles.
So this is not so much about politics as it is about character and obedience. Those things came together and marked the actions of Mordecai and Esther. And that turned them to places of influence that changed their situation and reached out to a nation within a nation. Character and obedience in leadership change society.