Source: TW
The pUru-s as per the mahAbhArata were the preeminent yayAtian clan who established the dynastic seat to which the bharata-s & their successors the kuru-s & pa~nchAla-s ascended.
In the R^igveda the pUru-s are one of the most prominent clans. They mentioned at least 21 times as a clan.
Of those 16 are what may be termed positive mentioned – i.e., the gods help them win.
3 are negative – where the gods cause them to lose or crush them.
3 are neutral mentions.
2 of the 3 negative mentions are by vasiShTha. They stand out against the backdrop of positive mentions.
The other negative mention is by the royal sage paruchChepa.
Both vasiShTha & paruChepa also mention them positively. I have not been able to fully understand this ambivalence, though some have claimed to resolve it. One possibility is that the positive pUru-s were the old rulers whose role in the religion is acknowledged; the negative ones were probably contemporaneous members of the clan, who were enemies of paruchChepa and sudAs