ekonavimse vimsatime
vrsnisu prapya janmani
rama-krsnav iti bhuvo
bhagavan aharad bharam
SB 1.3.23

Translation
In the nineteenth and twentieth incarnations, the Lord advented Himself as Lord Balarama and Lord Krsna in the family of Vrsni [the Yadu dynasty], and by so doing He removed the burden of the world.

Purport by Srila A.C. Bhaktivedanta Swami Prabhupada
The specific mention of the word bhagavan in this text indicates that Balarama and Krsna are original forms of the Lord. This will be further explained later. Lord Krsna is not an incarnation of the purusa, as we learned from the beginning of this chapter. He is directly the original Personality of Godhead, and Balarama is the first plenary manifestation of the Lord. From Baladeva the first phalanx of plenary expansions, Vasudeva, Sankarsana, Aniruddha and Pradyumna, expands. Lord Sri Krsna is Vasudeva, and Baladeva is Sankarsana.