במדבר פרק יח, כו וְאֶל-הַלְוִיִם תְדַבֵּר וְאָמַרְ תָ אֲלֵּהֶם כִי -תִקְחוּ מֵּאֵּת בְנֵּי-יִשְרָאֵּל אֶת-הַמַעֲשֵּר אֲשֶר נָתַתִי לָכֶם מֵּאִתָם בְנַחֲלַתְכֶם וַהֲרֵּמֹתֶם מִמֶנּוּ תְרוּמַת ה' מַעֲשֵּר מִן-הַמַעֲשֵּר:
The opinion of Rabbi Akiva is that the Maaser Rishon goes to the Levi and not to the Cohen. Rabbi Eleazor ben Azaryah maintains that the Cohanim are also eligible to receive the Maaser Rishon, since the Cohanim are also called Leviim (based on verse 21 which says ולבני לוי הנה נתתי כל מעשר, the addition of the “ו” indicates the addition of the Leviim to the Cohanim in receiving Maaser). From other places, Rabbi Yehoshua ben Levi and other Chachomim also maintain that the Cohanim are included in the tribe of Levi and are eligible for the Maaser Rishon.
Nevertheless, Rambam decides the law like Rabbi Akiva that Maaser Rishon should only be given to the Leviim. This needs investigation since in general the law is not decided like Rabbi Akiva against multiple other Rabbis. Yet later commentators do not seem to comment on this issue.
However, Rabbi Epstein says possibly since the main opponent of Rabbi Akiva is Rabbi Elazor ben Azaryah, this disagreement is considered as if Rabbi Akiva is arguing with only one Rabbi, and thus, we would decide the law in accordance with Rabbi Akiva.
One should be aware that in the times of Ezra a decree was made that all the Maaser would be given to the Cohanim (since the Leviim did not return from Exile with Ezra). Perhaps, Ezra relied on this opinion that the Cohanim are also considered Leviim, when he penalized the Leviim by removing the Leviim from receiving the Maaser Rishon, and transferred the Maaser to the Cohanim.
