John Gill 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
Here is a double reading: the "Keri", or marginal reading, is, "you knowest"; an appeal of the psalmist to God, who knew the thoughts of the wicked concerning him, and their devices against him; as Kimchi: but the Scripture reading is, I know; expressing his full persuasion and assurance

that the Lord will maintain the cause of the afflicted, [and] the
right of the poor ;
of his poor and afflicted people, that are afflicted within and without, by men and devils; and who are poor as to the things of this world, and poor in spirit, and sensible of their spiritual poverty, but rich in grace: the cause of these God will maintain against their oppressors, and right their wrongs, and avenge their injuries; this the psalmist knew, and was assured of from the word of God, from instances and examples in former times, and from his own experience, (Psalms 9:4) .