Rashi comments on the opening verses of Korach's rebellion, identifying the leaders who revolted and discussing their motivations.
דתן ואבירם. בִּשְׁבִיל שֶׁהָיָה שֵׁבֶט רְאוּבֵן שָׁרוּי בַּחֲנִיָּתָם תֵּימָנָה שָׁכֵן לִקְהָת וּבָנָיו הַחוֹנִים תֵּימָנָה, נִשְׁתַּתְּפוּ עִם קֹרַח בְּמַחֲלָקְתּוֹ — "אוֹי לָרָשָׁע אוֹי לִשְׁכֵנוֹ". וּמָה רָאָה קֹרַח לַחֲלֹק עִם מֹשֶׁה? נִתְקַנֵּא עַל נְשִׂיאוּתוֹ שֶׁל אֱלִיצָפָן בֶּן עֻזִּיאֵל, שֶׁמִּנָּהוּ מֹשֶׁה נָשִׂיא עַל בְּנֵי קְהָת עַל פִּי הַדִּבּוּר, אָמַר קֹרַח, אַחֵי אַבָּא אַרְבָּעָה הָיוּ, שֶׁנֶּאֱמַר (שמות ו') "וּבְנֵי קְהָת" וְגוֹ', עַמְרָם הַבְּכוֹר נָטְלוּ שְׁנֵי בָנָיו גְּדֻלָּה, אֶחָד מֶלֶךְ וְאֶחָד כֹּהֵן גָּדוֹל, מִי רָאוּי לִטֹּל אֶת הַשְּׁנִיָּה? לֹא אֲנִי שֶׁאֲנִי בֶּן יִצְהָר, שֶׁהוּא שֵׁנִי לְעַמְרָם, וְהוּא מִנָּה נָשִׂיא אֶת בֶּן אָחִיו הַקָּטָן מִכֻּלָּם, הֲרֵינִי חוֹלֵק עָלָיו וּמְבַטֵּל אֶת דְּבָרָיו. מֶה עָשָׂה? עָמַד וְכָנַס ר"נ רָאשֵׁי סַנְהֶדְרָאוֹת — רֻבָּן מִשֵּׁבֶט רְאוּבֵן שְׁכֵנָיו וְהֵם אֱלִיצוּר בֶּן שְׁדֵיאוּר וַחֲבֵרָיו וְכַיּוֹצֵא בוֹ, שֶׁנֶּאֱמַר נְשִׂיאֵי עֵדָה קְרִאֵי מוֹעֵד וּלְהַלָּן הוּא אוֹמֵר (במדבר א') "אֵלֶּה קְרוּאֵי הָעֵדָה" — וְהִלְבִּישָׁן טַלִּיתוֹת שֶׁכֻּלָּן תְּכֵלֶת, בָּאוּ וְעָמְדוּ לִפְנֵי מֹשֶׁה, אָמְרוּ לוֹ טַלִּית שֶׁכֻּלָּהּ שֶׁל תְּכֵלֶת חַיֶּבֶת בְּצִיצִית אוֹ פְטוּרָה? אָמַר לָהֶם חַיֶּבֶת, הִתְחִילוּ לִשְׂחֹק עָלָיו, אֶפְשָׁר טַלִּית שֶׁל מִין אַחֵר חוּט אֶחָד שֶׁל תְּכֵלֶת פּוֹטְרָהּ, זוֹ שֶׁכֻּלָּהּ תְּכֵלֶת לֹא תִפְטֹר אֶת עַצְמָהּ? (תנחומא):
ודתן ואבירם AND DATHAN AND ABIRAM — Because the tribe of Reuben had their place, when they encamped, in the South, thus being neighbours of Kohath and his sons, who, too, encamped in the South (cf. Numbers 3:29), they (the Reubenites) joined Korah in his quarrel. “Woe to the wicked, woe to his neighbour!” — And what induced Korah to quarrel with Moses? He was envious of the princely dignity held by Elzaphan the son of Uziel (Midrash Tanchuma, Korach 1) whom Moses had appointed prince over the sons of Kohath although this was by the express command of God (Numbers 3:30). Korah argued thus: “My father and his brothers were four in number — as it is said, (Exodus 6:18) “and the sons of Kohath were [Amram and Izhar and Hebron and Uziel]”. — “As to Amram, the eldest, his two sons have themselves assumed high dignity, one as king and the other as High Priest; who is entitled to receive the second (the rank next to it)? Is it not “I” who am the son of Izhar, who was the second to Amram amongst the brothers? And yet he has appointed as prince the son of his (Amram’s) brother who was the youngest of all of them! I hereby protest against him and will undo his decision”. — What did he do? He arose and assembled 250 men, fitted to be heads of the Sanhedrin, most of them of the tribe of Reuben who were his neighbours, viz., Elizur the son of Shedeur, (the prince of the tribe of Reuben; cf. Numbers 1:5), and his colleagues, and others of a similar standing, — for here it states (v. 2) that they were “princes of the congregation, those who were called to the assembly (קראי מועד)”, and there (in another passage) it states, (Numbers 1:16) “these were they who were called to the congregation (קרואי העדה)” (amongst whom was also Elizur the son of Shedeur; cf. Numbers 1:5 and Rashi on Numbers 1:16), — and he attired them in robes of pure purple wool. They then came and stood before Moses and said to him, “Is a garment that is entirely of purple subject to the law of Zizith or is it exempt”? He replied to them: “It is subject to that law”. Whereupon they began to jeer at him: “Is this possible? A robe of any different coloured material, one thread of purple attached to it exempts it, and this that is entirely of purple should it not exempt itself (i.e. ipso facto be exempt) from the law of “Zizith”? (Midrash Tanchuma, Korach 2).