In Megillat Esther, Mordechai is described as an איש ימיני, a right-winger. Why does he feel it necessary to defend his political positions in his megillah?
It's pretty simple, actually.
He's a "right" winger, meaning that his politics and the way he did things were "right" meaning "correct". He was looking out for the king's best interests. (He saved the king's life, afterall.)
In contrast, Haman was wrong. By attempting to destroy the Jews, he would have severed a significant source of the king's income. He wasn't looking after the king's interest. He was wrong and, therefore got "LEFT" over on a tree.
If the left wingers of his day were anything like the left wingers of our day, they would have been pushing to assimilate into the Persian society of their time. Like J Street nowadays, they complained that he should have bowed to Haman and not insisted on being religious. They also complained that he caused anti-Semitism by not surrendering to the cultural superstitions of their day.
Immediately preceding that he is described as a בן יאיר, a son of your ear. If so, you may have thought he's a socialist, as he wants your ear. Ka mashma lan איש ימיני. If so, what does בן יאיר mean? It must refer to the fact that he was a prominent member of the Sanhedrin, and, as such, always had people's ears when he spoke.
We can understand this based on the current political situation, where there are two Binyamins vying for the position of prime minister (the position that Mordechai eventually ended up with).
Mordechai, the Megillah tells us, was from Binyamin. But which one? The answer is that he's on the "Netan-yahu" side - recognizing everything he has was given to him by Hashem, rather than on the "Gantz" (Yiddish for "complete") side, thinking that he's complete as is.
In the final 3 verses of the Megilla, we see that Achashverosh raises taxes on his kingdom.
וַיָּשֶׂם֩ הַמֶּ֨לֶךְ אחשרש [אֲחַשְׁוֵר֧וֹשׁ ׀] מַ֛ס עַל־הָאָ֖רֶץ וְאִיֵּ֥י הַיָּֽם׃
King Ahasuerus imposed taxes on the mainland and the islands.
Then he puts in Mordechai as Prime Minister.
וְכָל־מַעֲשֵׂ֤ה תָקְפּוֹ֙ וּגְב֣וּרָת֔וֹ וּפָרָשַׁת֙ גְּדֻלַּ֣ת מָרְדֳּכַ֔י אֲשֶׁ֥ר גִּדְּל֖וֹ הַמֶּ֑לֶךְ הֲלוֹא־הֵ֣ם כְּתוּבִ֗ים עַל־סֵ֙פֶר֙ דִּבְרֵ֣י הַיָּמִ֔ים לְמַלְכֵ֖י מָדַ֥י וּפָרָֽס׃
All his mighty and powerful acts, and a full account of the greatness to which the king advanced Mordecai, are recorded in the Annals of the Kings of Media and Persia.
Nevertheless some Jewish people still blamed Mordechai for the higher taxes, which is why he was popular only with the majority of the Jews, but not all.
כִּ֣י ׀ מָרְדֳּכַ֣י הַיְּהוּדִ֗י מִשְׁנֶה֙ לַמֶּ֣לֶךְ אֲחַשְׁוֵר֔וֹשׁ וְגָדוֹל֙ לַיְּהוּדִ֔ים וְרָצ֖וּי לְרֹ֣ב אֶחָ֑יו For Mordecai the Jew was second to King Ahasuerus and was great to the Jews and popular with most of his brethren
Thus the Megilla informs us at the beginning that Mordechai was a right winger who didn't want to impose taxes, but couldn't stop Achashverosh from imposing them.