At the climax of the eloquent plea to spare Binyamin from a lifetime of slavery, Yehudah utters the words that melt the viceroy’s heart:
“And it shall be, when he [Yaakov] sees that the lad is not [with us], he shall die; and your servants shall have brought the gray hair of your servant our father in grief to the grave. Because your servant guaranteed (ARAV) the lad from my father, saying, ‘If I do not bring him to you then I will be sinning to my father all the days’. And now, please let your servant remain instead of the lad, as a servant to my lord, and let the lad go up with his brothers. For how can I go up to my father if the lad is not with me? Lest I will witness the evil that will befall my father!” (Bereishit 44:31-34).

The viceroy then reveals his true identity as Yosef.

Yehudah does not disclose what occurred “behind-the-scenes” with Yaakov, but we are aware of the difficult deliberations before Yaakov allowed his sons to return to Egypt with Binyamin. Reuven, the first-born, took the initiative:
And Reuven said to his father, saying, “Execute my two sons if I do not bring him to you. Put him in my hand and I will restore him to you” (Bereishit 42:37).
Yaakov rejected this offer, relenting only when all the provisions were depleted.

It was then that Yehudah made his proposal:
And Yehudah said to Yisrael his father, “Send the lad with me and let us get up and go, so we will live and not die, both we and you and our children. I will guarantee him (E’ERVENU); from my hand shall you seek him. If I do not bring him to you and present him before you, then I will have sinned before you all the days. For had we not tarried, then by now we could have returned twice!” (43:8-10).
As Rashi says, Yehudah meant that he would suffer, even in the next world, if he would not return Binyamin. It was when Yehudah guaranteed (E’ERVENU . . . ARAV) Binyamin’s safety that Yaakov finally agreed.

But, what is the nature of this pledge, and what effect does it have on the future?

The Talmud (Bava Batra 173b) discusses Yehudah’s promise as the possible source for the halachic principle of arevut: surety, an enforceable verbal commitment to others. In its most familiar form, an arev undertakes to repay a loan on behalf of the debtor. On the one hand, Yehudah might not be a true arev, since he is both the surety and the debtor. On the other hand, Yehudah stands as surety for Reuven’s commitment to Yaakov.

What is certain is that this pledge forges a bond between Yehudah and Binyamin that is the key to Yosef’s reunion with his family.

This bond has historic ramifications:

• When Yishai’s sons are off at war against the Philistines, he sends his youngest son, David, to bring them provisions, saying:
  “…and look how your brothers fare, and their pledge (ARUBATAM) shall you bring” (Shmuel I 17:18).
  The Sages explain (Midrash Shmuel 20:8; Yalkut Ha’mechiri Tehillim 48:4 from Tanchuma) that David must make good on the guarantee (AREVUT) made by his ancestor Yehudah regarding Binyamin, the ancestor of King Shaul: By defeating the Philistine champion Goliat, David will save Shaul.

• Binyamin is the first tribe of the house of Yosef to follow David, despite their relation to Shaul (Shmuel II 3:19, 19:21; Melachim I 11:32; see Redak).

• When he is pursued by Shaul, supporters of David come
  from the children of Binyamin and Yehudah (Divrei HaYamim I 12:17).

• Yerov’am ben-Nevat rebels against the house of David, leading to a division within the kingdom, but:
  And when Rechav’am came to Jerusalem, he assembled all the house of Yehudah, with the tribe of Binyamin, one hundred eighty thousand chosen men, warriors, to fight against the house of Yisrael, to return the kingdom to Rechav’am son of Shlomo (Melachim I 12:21).

• After the division, the two tribes are one:
  . . . none was left but the tribe of Yehudah alone (Melachim II 17:18).
  On this, Redak says: “And the tribe of Binyamin was secondary to it and included in it.” (See also Divrei HaYamim II 11:1,3,10,12,23; 15:2,8,9; 25:5; 31:1; 34:9.)

• This union continues after the exile to Babylon, when
  the heads of the families of Yehudah and Binyamin arose, together with Kohanim and Leviim, all those whose spirit G-d stirred, to go up to build the house of Hashem in Jerusalem (Ezra 1:5; see also 4:1; 10:9).
  The Beit HaMikdash was partially in the territory of Yehudah and partially in Binyamin (Zevachim 53b).

• The miracles of Purim came about through Esther and Mordechai, who is called both a Yehudi and a man of Binyamin (Esther 2:5) .

The Talmud (Sukkah 27b; Tosefta Sukkah 1:5) notes other similarities between these two tribes:

• Yehudah and Binyamin produced the only kings who ruled over the united 12 tribes: Shaul from Binyamin, and David and Shlomo from Yehudah.

• The kings of these two tribes were chosen by prophets: Shaul and David by Shmuel. (There is a view that Yerov’am ben Nevat, who was anointed by Achiya HaShiloni, was from the tribe of Binyamin.)

In “Mah Bein Shaul L’David”, R. Yehoshua Bachrach compares the two:
Binyamin is the introvert, characterized by modesty, silence and restraint. The youngest child, he is orphaned at birth, and is sheltered and granted privileges to compensate for this hardship. He becomes the smallest tribe, behaving like a pampered and troubled child, and providing the meteoric and unstable reign of Shaul. Yehudah is the extrovert, characterized by action, garrulousness, and natural leadership. He earns his privilege through his own efforts and deliberations, learning as well from his errors. He becomes the largest tribe, behaving like a responsible and family-oriented child, and providing the stable and steady monarchy of the house of David.

It is altogether fitting that Yehudah and Binyamin remain united.