The Mahopanishad line
They say Narayana was alone. There were not Brahma, Shiva
seems to be the very foundation of Hari Sarvottamatvam for Madhwites and Srivaishnavites.
Mahabharata proclaims Vishnu's supremacy profusely - but other Gods are praised as Brahman also and Vaishnavites seem to need the Mahopanishad line to claim that whenever some other God is praised as Brahman, it only means Vishnu.
This line allows them to claim that even in Rig Veda, ONLY Vishnu's supremacy is proclaimed.
Are Shruti statements that allegedly rank Vedic Devas considered to proclaim Vishnu's supremacy equally strongly as Mahopanishad, or are these just confirmatory of Mahopanishad?
Agnir vai devānām avamo Viṣṇuḥ paramas, tadantareṇa sarvā anyā devatā
which Vaishnavites translate as
Agni is the lowest of the deva-s and Vishnu is the highest. Between them are all other deva-s.
When about to consecrate himself, he offers to Agni and Visnu on eleven potsherds; all the deities are Agni; the sacrifice is Visnu; verily he lays hold of the deities and the sacrifice; Agni is the lowest of the deities, Visnu the highest; in that he offers to Agni and Visnu on eleven potsherds, the sacrificer envelops the gods on both sides and wins them.
Thanks to @also ran for the references.