Plead my cause, O Lord - The word “plead” means, properly, to argue in support of a claim, or against the claim of another; to urge reasons for or against; to attempt to persuade one by argument or supplication; as, to plead for the life of a criminal, that is, to urge reasons why he should be acquitted or pardoned; and then, to supplicate with earnestness in any way. The original word used here - רוב rûb - means to contend, strive, quarrel; and then, to contend before a judge, to manage or plead a cause. The idea here is, that the psalmist desires that God would undertake his cause against those who had risen up against him, as if it were managed before a tribunal, or before a judge, and God should be the advocate. The same word is used, in another form, in the other member of the sentence - “with them that strive - יריבי yârı̂ybāy - against me.” The idea is, that they were “pleading” against him, or were urging arguments, as it were, before a tribunal or a judge, why he should be condemned. They were his bitter opponents, engaged in bringing all manner of false accusations against him, and seeking his condemnation. The psalmist felt that he could not manage his own cause against them; and he, therefore, pleads with God that He would interpose, and stand up for him.
Fight against them that fight against me - The same idea substantially occurs here as in the former member of the verse. It is a prayer that God would undertake his cause; that He would exert His power against those who were opposed to him.