The Mordechai Megillah (776) says when Taanis Ester is on Shabbos we do the fast earlier, unlike other fasts which are postdated since it is not a fast for bad things that happened. What does he mean? The Rambam Taanis Ch. 5 says יֵשׁ שָׁם יָמִים שֶׁכָּל יִשְׂרָאֵל מִתְעַנִּים בָּהֶם מִפְּנֵי הַצָּרוֹת שֶׁאֵרְעוּ בָּהֶן and he delineates the 4 fasts. In halacha 5 he says וְנָהֲגוּ כָּל יִשְׂרָאֵל בִּזְמַנִּים אֵלּוּ לְהִתְעַנּוֹת וּבִּשְׁלוֹשָׁה עָשָׂר בַּאֲדָר, זֵכֶר לְתַעֲנִית שֶׁנִּתְעַנּוּ בִּימֵי הָמָן, שֶׁנֶּאֱמַר "דִּבְרֵי הַצּוֹמוֹת וְזַעֲקָתָם. We see from the Rambam that Taanis Ester is different from the other fast days which are to remember the hardships of the day, it is a zecher to the fast in the time of Haman. It is not a fast day to remember hardships.
The Shulchan Aruch (550:4) says that the Shabbos before a fast day that is up to the acceptance of Klal Yisrael it is announced that that the fast is that week and he lists Taanis Eeter as an exception. Why? The Gra says since it is not a reshus but is mandated as the Rambam evokes the possuk דִּבְרֵי הַצּוֹמוֹת וְזַעֲקָתָם. This makes it seem that Taanis Ester is stronger than the other fast days but on the other hand we find the Rema (687) says that it is more lenient and one in even a small measure of pain can break their fast? Rav Zolty (Mishnas Yaavetz #76) says that the difference between Taanis Ester and the other fast days is that the other fast days have the status of a תענית (which has laws beyond not eating of being in a somber state etc.) but Taanis Ester doesn't have the status of a יום תענית it is just a prohibition of eating and drinking. The Rishonim ask how can we fast Taanis Ester if Purim is part of Megillas Taanis which forbids the previous day in fasting as well? The Raavad (cited in Ran Taanis 7a in Rif) says it is not an issue since the fast it זכרון לנס שנעשה בו ויש סמך בפסוק דברי הצומות. Rav Zolty explains he means this idea that it is not a day that is a chalos taanis but as part of Purim we also commemorate the fast. This may be the intent of the Rambam as well who cited the possuk as the basis for the custom to fast.
Rav Zolty understands the intent of the Gra is that this fast is part of the establishment of Purim itself. Just as Purim is forever, so too is Taanis Ester. A support for the idea the assertion that Taanis Ester will remain in the days of Moshiach is supported by the Binyan Shlomo (siman 56.) He proves from Rishonom that they fasted during Bais Sheni. He asks why we say מזמור לתודה on Taanis Ester if they fasted during Bais Sheni they wouldn't have offered the korban as it would diminish the time it could be eaten? However, we see he understands that Taanis Ester applies even בזמן שלום. Rav Zolty says this is what the Rambam says (end of Ch. 2 Laws of Megillah) אַף עַל פִּי שֶׁכָּל זִכְרוֹן הַצָּרוֹת יִבָּטֵל, שֶׁנֶּאֱמַר "כִּי נִשְׁכְּחוּ הַצָּרוֹת הָרִאשֹׁנוֹת וְכִי נִסְתְּרוּ מֵעֵינָי" (ישעיהו סה, טז) - יְמֵי הַפּוּרִים לֹא יִבָּטְלוּ. What is the contrast? The Rambma is contrasting that the tzarah preceding Purim, the Taanis will remain as well because the Taanis is part of the establishment of Purim.
Rav Tzvi Ryzman cites a Shitas Gaonim that even where there is a second Adar there is a fast of Ester in the first Adar as well. It is clear they hold that the taanis is not tied to the holiday of Purim. This would be not like the idea of Rav Zolty.
Why is Taanis Ester so significantly tied to Purim? The Rambam writes (end of Sefer Mitzvos Hakatzar) the point of Purim is to demonstrate that Hashem answers our prayers. The point of Taanis Ester is to commemorate the day of tefillah that Klal Yisrael had. It is part of the message of Purim itself and therefore is part of the days of Purim. The Tur (693) says כתב רב עמרם ז"ל: מנהג בשתי ישיבות ליפול על פניהם כיון שהוא יום נס ונגאלו בו, צריכין אנו לבקש רחמים שיגאלנו באחרונה כבראשונה. In light of this Rambam it is understood that since the essence of the day is a day of prayer it isa befitting to ask for the ultimate geulah (it is a chiddush to me that tachnun is considered to be asking for the future geulah.)





