mitzvah of bigdei kehuna

1) Rambam lists wearing bigdei kehuna as a mitzvas aseh in Sefer ha'Mitzvos aseh #33, while BH"G counts it as a lav, not to do avodah without wearing the proper bigdei kehuna.  

Ramban, in his hasagos to the Sefer haMitzvos, questions why the Rambam counts this as a separate mitzvah.  The rule, which the Rambam himself agrees to and sets down, is that you don't count parts of existing mitzvos as new independent mitzvos.  Wearing bigdei kehuna, argues Ramban, is part and parcel of the mitzvah of doing avodah; it's just a hechsher mitzvah.  One of the steps of properly performing a rite in the mikdash is wearing the appropriate bigdei kehuna for the job. (See R' Chaim in the stencils.)

We read in our parsha:

 וְעָשִׂ֥יתָ בִגְדֵי־קֹ֖דֶשׁ לְאַהֲרֹ֣ן אָחִ֑יךָ לְכָב֖וֹד וּלְתִפְאָֽרֶת

 וְאַתָּ֗ה תְּדַבֵּר֙ אֶל־כׇּל־חַכְמֵי־לֵ֔ב אֲשֶׁ֥ר מִלֵּאתִ֖יו ר֣וּחַ חׇכְמָ֑ה וְעָשׂ֞וּ אֶת־בִּגְדֵ֧י אַהֲרֹ֛ן לְקַדְּשׁ֖וֹ לְכַהֲנוֹ־לִֽי

Rashi comments: 

לקדשו לכהנו לי – לקדשו, להכניסו בכהונה על ידי הבגדים שיהא כהן לי. ולשון כהונה שֵירוּת היא

We see from Rashi that there are two functions to the bigdei kehuna: 1) it sanctifies the kohen להכניסו בכהונה, to take on the role of kohen ; 2) it allows the kohen to do avodah and serve in the mikdash.  As Maharal writes in Gur Aryeh, ולפי זה ״לקדשו״ על הכנסתו לכהונה, ו״לכהנו״ רוצה לומר לכהן לי אחר כך:

The Lev Sameich on the Sefer haMitzvos is medayek the same from the Rambam in Klei haMikdash 10:4

מצות עשה לעשות בגדים אלו ולהיות הכהן עובד בהן שנאמר ועשית בגדי קדש. ואת בניו תקריב והלבשתם כתנות

The mitzvah, says Rambam, is to make the bigdei kehuna AND for the kohen to do avodah wearing them -- two separate ideas.  As Ramban al haTorah himself writes, the bigdei kehuna were like royal garments.  Even when the kohen was not doing avodah, the bigdei kehuna served לְכָב֖וֹד וּלְתִפְאָֽרֶת to distinguish them from ordinary people.  

Therefore, the Rambam counts bigdei kehuna as its own mitzvah, separate and apart from the need to wear them as a hechsher miztvah to do avodah.

2) The Ohr Sameich (Isurei Biah 4:8) asks: m'doraysa only a chatzitza that covers most of the body invalidates tevilah.  Even though the Torah says "v'rachatz es KOL bisaro ba'mayim," as long as most of the body comes in contact with the mikveh, rubo k'kulo.  Why then is it that even just one thread that comes between bigdei kehuna and the kohen's body is an invalidating chatzitza (Zevachim 19)?  The mitzvah of wearing bigdei kehuna is not complete until all the bigdei kehuna are donned.  Why not say rubo k'kulo on the mitzvah of levisha, and so long as most of the kohen's body is covered by the bigdei kehuna, a small chatzitza should not matter?

Take a look at his answer!