John Gill Commentary Psalms 35:1

"Strive thou, O Jehovah, with them that strive with me: Fight thou against them that fight against me." — Psalms 35:1 (ASV)

Plead [my cause], O Lord, with them that strive with me
Meaning Saul and his courtiers; concerning whom he elsewhere desires that the Lord would judge between them, plead his cause, and deliver him; as he accordingly did, and maintained it, and the righteousness of it, (1 Samuel 24:12–15) (Psalms 9:4) . So Christ pleaded not his own cause as man, but committed himself to him that judgeth righteously; and his people leave their cause with him, who is their advocate, and is able to plead it thoroughly; and does plead it against wicked and ungodly men, who unrighteously charge them; against Satan the accuser of the brethren, who stands at their right hand to resist them; and against their own hearts, and the sins of them, which lust and war against them, and condemn them;

fight against them that fight against me :
so the Lord is sometimes represented as a man of war, and Christ as a warrior fighting for the saints; and safe are they on whose side he is; but miserable all such who are found fighters against him and his; for none ever opposed him and prospered.