Albert Barnes Commentary Psalms 140:12

"I know that Jehovah will maintain the cause of the afflicted, And justice for the needy." — Psalms 140:12 (ASV)

I know that the Lord will maintain the cause of the afflicted - See the notes at Psalms 9:4. The psalmist here doubtless refers primarily to himself, as having a confident belief that the Lord would maintain his cause, or would defend him. At the same time, he makes the statement general, implying that what would be done to him would be done to all in similar circumstances.

The idea is that God, in all his attributes, in all his providential arrangements, in all his interpositions on earth, would be found to be on the side of the oppressed, the afflicted, and the wronged. He has no attribute that can take part with an oppressor or a wrongdoer. The wicked cannot come to him with the belief that he will be on their side: the righteous—the oppressed, the afflicted—can.

And the right of the poor - He will defend the right of the poor. Literally, The judgment of the poor. That which will be just and right in their case.