when did ahron become a kohein?

A comment to last week’s post suggested that Moshe did not become kohein because he had slain a Mitzri; Ahron had never taken anyone’s life. I replied that Chazal (Zevachim 102, cited by Rashi) comment on the words “ha-lo Ahron achicha haLevi” (Shmos 4:14) that Moshe had in fact been destined to be a kohein, but because of his reluctance to accept the mission of Redeemer, his brother Ahron was chosen in his stead – not because Moshe shed blood. The counterargument is that the election of Ahron was determined by multiple causes.

Chazal (Sanhedrin 7) justify Ahron’s choice to fashion the eigel by citing the pasuk promising destruction “Im yehareig b’mikdash Hashem kohein v’navi”, if a kohein and prophet be killed. Chur, the son of Miriam, who was a prophet, had already been killed by the mob as he tried to oppose them. Had Ahron stood in the way and been killed as well, the punishment to the nation would have been far worse. Therefore, Ahron decided to play along and fashion an eigel, doing his best to stall the people and mitigate the damage.

The Maharasha asks: why does the gemara consider Ahron as having the status of kohein? His election as kohein does not occur until later parshiyos. Maharasha answers that Ahron was a bechor, a first born, and before the election of Levi’im the avodah was entrusted to the bechorim.

We can perhaps answer the Maharasha’s question if we assume that Ahron’s election occurred earlier during the episode described in Parshas Shmos referenced above (Margoliyas haYam). Ahron was not just the kohein in-waiting, but actually had the status of kohein from that early moment onward. This would also resolve the question according to the Midrashic sources that say Miriam was the first-born and not Ahron.