What is "u'va'Midbar Asher Ra'isa ... " referring to?
Rashi: It is a continuation of the previous Pasuk - 'Hashem will fight for you like all that He did for you in Egypt ... and subsequently, in the desert ... '.
Seforno: In answer to Yisrael's claim "be'Sinas Hashem osanu" in Pasuk 27 ? Refer to 1:27:2:2 - Moshe pointed out that, had Hashem wanted to kill them, He would have left you to the mercy of the snakes and scorpions that abounded in the desert. Moreover, the notion that He hated them, Moshe dispelled in Pasuk 33, when he reminded them how He sought locations for them to rest in the unhospitable desert ?- a sign of His love and the compassion that He felt towards them.
Targum Yonasan: "u'va'Midbar asher Ra'isa ... " refers to the highly venomous snakes that they saw in the desert, and which the Cloud had killed. 1
Oznayim la'Torah: It is in answer to Yisrael's claim "be'Sinas Hashem osanu" in Pasuk 27. 'How could you say such a thing when you saw how Hashem led you in the desert like a father leads a son - even as He punished you". 2
What is Moshe referring to when he states "Ka'asher Yisa Ish Es Bno ..."?
Rashi: It refers to Hashem's protection of Yisrael. Refer to Sh'mos, 14:20:1:1*.