WHEN KOHANIM WASH FOR BIRKAS KOHANIM [Birkas Kohanim:washing]
(Beraisa): Shechitah must always be Tekef to (immediately after) Semichah, except for Asham Metzora, for which Semichah is by Sha'ar Nikanor. The Metzora cannot enter the Azarah until the blood of the Asham is put on him and the blood of the Chatas is Nizrak.
Inference: Bi'ah b'Miktzas is Bi'ah. If it were not, he could stick his hands in and do Semichah!
Rejection (Rav Yosef): The Tana is R. Yosi b'Rebbi Yehudah, who says that Tzafon (the north part of the Azarah) is far from the end of Sha'ar Nikanor.
Sotah 38b (R. Yehoshua ben Levi): Any Kohen who does not ascend (to the Duchan, where the Kohanim bless) during the Berachah of Retzeh (in the repetition of Shemoneh Esreh) may not bless - "Aharon lifted his hands to the people and blessed them, and descended from offering..." Just like there he blessed amidst Avodas ha'Mikdash, also Kohanim must ascend to bless during the Berachah of Avodah.
Objection: R. Ami and R. Asi blessed, even though they did not ascend during Retzeh!
Answer: They uprooted their feet (started walking) during Retzeh, just they did not arrive until after Retzeh.
Support #1 (R. Oshaya): The Kohen is forbidden to bless only if he did not uproot his feet during Retzeh.
Support #2 (Mishnah): If a Shali'ach Tzibur is the only Kohen in the Beis ha'Keneses, if he is sure that he can bless and resume the repetition of Shemoneh Esreh, he may bless.
Question: He does not do Akirah (uproot his feet during Retzeh)!
Answer: It suffices that he moves his feet a drop towards the Duchan.
The same applies to R. Ami and R. Asi.
39a (R. Yehoshua ben Levi): A Kohen who did not wash his hands may not bless - "lift your hands in Kedushah and bless Hash-m."
Berachos 42a (R. Chiya bar Ashi): Shechitah must be Tekef to Semichah, Tefilah (Shemoneh Esre) must be Tekef to Ge'ulah (Go'al Yisrael), and Berachah must be Tekef to washing.
Rambam (Hilchos Tefilah 15:1): Tum'ah of the hands prevents a Kohen from giving Birkas Kohanim.
Rambam (5): A Kohen who did not wash his hands may not Duchan. Rather, he washes his hands until the wrist, like Kohanim wash for Avodah, and then blesses.
Rashi (33a DH Merchak): Kodshei Kodoshim must be slaughtered in Tzafon. The Tana holds that Tzafon is only to the north of the Mizbe'ach, which is 22 Amos west of Sha'ar Nikanor. Therefore, it is impossible to do Semichah from the Ezras Nashim, and Tekef l'Semichah Shechitah in the north.
Tosfos (Sotah 39a DH Kol): Rashi wrote that his Rebbi said that he need not wash again. It seems that this was copied from a Talmid's comment, for Rashi connotes that he must wash just before ascending. Also, the Gemara learns from "lift your hands in Kedushah and bless Hash-m" that one must wash right before Birkas Kohanim. Our Gemara says that the verse discusses Birkas Kohanim. The Yerushalmi holds that it discusses washing for Birkas ha'Mazon. Berachos 42a connotes similarly, but in any case it teaches that one must wash before the Berachah that the verse discusses, and our Gemara holds that it discusses Birkas Kohanim. Also, if we discuss one who did not wash (from when he woke up) in the morning, why do we discuss a Kohen giving Birkas Kohanim? One may not say Divrei Torah until washing his hands (Berachos 11b)! One must go up to a Mil (about a kilometer) for water to wash for Kri'as Shema. Why should Birkas Kohanim be any different? Rather, we require washing Tekef before blessing. We can learn the Shi'ur of Tekef from Shechitah, which must always be Tekef to Semichah, except for Asham Metzora, in which Semichah is by Sha'ar Nikanor. This shows that from Sha'ar Nikanor to where they slaughter in not considered Tekef. In Midos, it connotes that this is more than 22 Amos.
Shulchan Aruch (OC 128:8): When the Shali'ach Tzibur begins Retzei, every Kohen in the Beis ha'Keneses uproots his feet to go to the Duchan. Even if he will not get there until the Shali'ach Tzibur finishes Retzei, this is fine. If he did not uproot his feet during Retzei, he may not go to Duchan.
Beis Yosef (DH v'Yitol): The Rambam connotes that if his hands were Tamei, e.g. he did not wash in the morning, he must wash now, but if he washed in the morning, and does not know that they became Tamei, he need not wash again. If not, he should have said 'even if his hands are not dirty and he does not know of any Tum'ah, he may not Duchan until he washes', like he wrote in Hilchos Berachos (6:1) about washing for eating. Also, he mentioned Tum'ah of the hands. This implies that if he washed in the morning, and he does not know of any Tum'ah, he need not wash again. Also for Avodah, a Kohen washes once in the morning for the entire day and night! The custom in Mitzrayim is not to wash at the time of Birkas Kohanim. They rely on Netilas Yadayim in the morning. However, Rashi explains that a Kohen who did not wash before ascending the Duchan may not Duchan. This connotes that even if he washed in the morning, and knows that his hands are Tahor, he must wash again shortly before ascending the Duchan. Orchos Chayim cites the Rambam's son to say that a Kohen may rely on Netilas Yadayim he did in the morning. The Tur holds like Tosfos. Since he must wash, he must bless on washing, just like we bless on washing for a meal. Tosfos asked that one may not say any Divrei Torah until washing in the morning! To defend the other opinion, I say that the Chidush of Birkas Kohanim pertains to one who washed and ceased to guard his hands. He need not wash for Divrei Torah or Kri'as Shema. Since he does not know that they are Tamei, we rely on the Chazakah that they are Tahor. He must wash for Birkas Kohanim. Alternatively, if one has no water, for Divrei Torah or Kri'as Shema, it suffices to clean his hands in any way. To Duchan, he must wash with water. This is the custom of the Ashkenazim, to wash even if he already washed. I heard that in Sefard the Leviyim wash the Kohanim's hands. I did not know the source until I found it in the Zohar.
Magen Avraham (10): Even if he was not Oker during Retzei due to Ones, e.g. he was praying (Shemoneh Esre), he does not Duchan. If he ascended to the Duchan, he does not descend. Starting to walk from his house to the Beis ha'Keneses is not called Akirah (Radvaz). I say that it is, if his house is next to the Beis ha'Keneses.
Kaf ha'Chayim (53): The Pri Chodosh says that if he was praying, this is not Ones. It would have sufficed to uproot his legs a little. In Sotah, we say that a Shali'ach Tzibur may Duchan if he is confident that he can resume praying afterwards. We asked that he does not do Akirah, and answered that he moves a drop towards the Duchan. Ginas Veradim (1:22) said that the Gemara connotes that if he did not do Akirah during Retzeh, even if he ascended, he descends. He concluded that the Radvaz says that he does not descend only if he was Shogeg, but not if he was warned not to ascend and disobeyed. Olas Tamid infers that the Radvaz says that he does not descend only if he was not Oker in Retzeh due to Ones. He was unsure what to do in practice. Magen Giborim says that if it was not Ones, he descends.
Mishnah Berurah (25): L'Chatchilah he must be Oker when the Shali'ach Tzibur begins Retzeh. If he did not, as long as the Shali'ach Tzibur did not finish Retzeh, it is fine.
Mishnah Berurah (27): It suffices to reach the Duchan before the Shali'ach Tzibur finishes Modim, as long as he was Oker during Retzeh. The Shulchan Aruch mentioned Retzeh, because the Reisha (when he starts) discussed Retzeh. The primary Akirah is after washing, for then he can go to the Duchan. Therefore, they should go to wash before Retzeh, and at Retzeh they start to walk to the Duchan. B'Di'eved, if he did not wash before Retzeh, and if he goes to wash the Shali'ach Tzibur will finish Retzeh, even so he should start to walk to the Duchan, and afterwards wash there (i.e. they will bring to him water near the Duchan).
Mishnah Berurah (28): Ateres Zekenim holds that even if he was Oker to go outside to wash, this is not called Akirah, even though washing enables Birkas Kohanim, for Akirah must be towards the Duchan, and his Akirah was to the outside. Therefore, the custom is that if the Shali'ach Tzibur reached Retzeh and the Kohanim still did not come to the Aron, he waits until they come. We are concerned lest they still did not start walking from where they washed towards the Duchan. The Poskim say that if he started to walk from his house to the Beis ha'Keneses and found (the Shali'ach Tzibur) after Retzeh, even though surely he started walking before Retzeh, this is not considered Akirah, since he did not uproot with intent for the Duchan. If he had this intent, it helps, and he blesses. Some say that even in this case he goes up only if his house is near the Beis ha'Keneses and he heard the Shali'ach Tzibur begin Retzeh.
Kaf ha'Chayim (55): Maharikash permits a Kohen who entered the Beis ha'Keneses during Retzeh to Duchan. The Pri Chodosh refutes this from the Radvaz. Ginas Veradim (22) suggests that the Radvaz discusses one who did not intend to Duchan in the Beis ha'Keneses, and Maharikash discusses one who came with intent to Duchan.
Kaf ha'Chayim (48): The Gemara and Rashi connote that Akirah may be any time during Retzeh. The Pri Chodosh says that the Shulchan Aruch and Tur connote that it must be at the beginning; this is wrong. The Birkei Yosef says that also the Shulchan Aruch and Tur permit any time during Retzeh, just l'Chatchilah it should be at the beginning.
Kaf ha'Chayim (50): Even a Kohen who prays where he gives Birkas Kohanim must uproot his feet during Retzeh.
Kaf ha'Chayim (52): Maharam Mintz was unsure if when others tell him to Duchan, if he goes up even if he did not do Akirah during Retzeh. Eliyahu Rabah leans to say that he must. If so, if a Kohen forgot to do Akirah during Retzeh, he should go outside, lest someone tell him to go up and he enter a Safek.