SB 4.21.50

nātyadbhutam idaṁ nātha
 tavājīvyānuśāsanam
prajānurāgo mahatāṁ
 prakṛtiḥ karuṇātmanām
Synonyms: 
na — not; ati — very great; adbhutam — wonderful; idam — this; nātha — O lord; tava — your; ājīvya — source of income; anuśāsanam — ruling over the citizens; prajā — citizens; anurāgaḥ — affection; mahatām — of the great; prakṛtiḥ — nature; karuṇa — merciful; ātmanām — of the living entities.
Translation: 
Our dear lord, it is your occupational duty to rule over your citizens. That is not a very wonderful task for a personality like you, who are so affectionate in seeing to the interests of the citizens, because you are full of mercy. That is the greatness of your character.
Purport: 

A king’s duty is to give protection to his citizens and levy taxes from them for his livelihood. Since the Vedic society is divided into four classes of men — the brāhmaṇas, kṣatriyas, vaiśyas and śūdras — their means of livelihood are also mentioned in the scriptures. The brāhmaṇas should live by spreading knowledge and should therefore take contributions from their disciples, whereas a king should give protection to the citizens for their development to the highest standard of life, and he can therefore levy taxes from them; businessmen or mercantile men, because they produce foodstuffs for the whole of society, can take a little profit from this, whereas the śūdras, who cannot work as either brāhmaṇas, kṣatriyas or vaiśyas, should give service to the higher classes of society and be provided by them with a supply of the necessities of life.

The symptom of a qualified king or political leader is mentioned herein: he must be very merciful and compassionate to the people and see to their prime interest, which is to become elevated devotees of the Supreme Personality of Godhead. Great souls are naturally inclined to do good to others, and a Vaiṣṇava especially is the most compassionate and merciful personality in society. Therefore we offer our respects to a Vaiṣṇava leader as follows:

vāñchā-kalpatarubhyaś ca
 kṛpā-sindhubhya eva ca
patitānāṁ pāvanebhyo
 vaiṣṇavebhyo namo namaḥ

Only a Vaiṣṇava leader can fulfill all the desires of the people (vāñchā-kalpataru), and he is compassionate because he is the contributor of the greatest benefit to human society. He is patita-pāvana, the deliverer of all fallen souls, because if the king or the head of the government follows in the footsteps of the brāhmaṇas and Vaiṣṇavas, who are naturally leaders in missionary work, the vaiśyas will also follow in the footsteps of the Vaiṣṇavas and brāhmaṇas, and the śūdras will give them service. Thus the entire society becomes a perfect human institution for combined progress to the highest perfection of life.