Here I want to say something. All the Pandavas were referred as Paarthas in the main Sanskrit epic. There are words like Paarthaanaam at several places which is a vahu vachan (plural form) and clearly refers to all the sons of Pritha.
As for Krishna calling Arjun 'Paartha', I can show you many verses where Krishna is addressing Yudhishthir too as 'Paartha'. See this one where He is praising Yudhishthir:
Daanam Cha Satyam Cha Tapo Cha Raajan
Shraddhaa Cha Shaantih Cha Dhritih Kshamaa Cha
Avapya Raashtrani Vasuni Bhogan
Esa Para Paartha Sadaa Ratih Te.
[O son of Pritha, despite of being the ruler of a kingdom and having wealth and means of enjoyment, you always have been attracted only to charity, truth, austerity, respect (to others), peace, patience and forgiveness.]
Edited by DharmaPriyaa - 8 years ago