Biblical Commentary on the Old Testament, by Carl Friedrich Keil and Franz Delitzsh, [1857-78], at sacred-texts.com
Cry of a Persecuted One for Help
This short Psalm, placed after Ps 69 on account of the kindred nature of its contents (cf. more especially v. 6 with Psa 69:30), is, with but few deviations, a repetition of Psa 40:14. This portion of the second half of Ps 40 is detached from it and converted into the Elohimic style. Concerning להזכּיר, at the presentation of the memorial portion of the mincha, vid., Psa 38:1. It is obvious that David himself is not the author of the Psalm in this stunted form. The לדוד is moreover justified, if he composed the original Psalm which is here modified and appropriated to a special liturgical use.
We see at once at the very beginning, in the omission of the רצה (Psa 40:14), that what we have here before us is a fragment of Ps 40, and perhaps a fragment that only accidentally came to have an independent existence. The להצּילני, which was under the government of רצה, now belongs to הוּשׁה, and the construction is without example elsewhere. In Psa 70:3 (= Psa 40:15) יחד and לספּותהּ are given up entirely; the original is more full-toned and soaring. Instead of ישׁמּוּ, torpescant, Psa 70:4 has ישׁוּבוּ, recedant (as in Ps 6:11, cf. Psa 9:18), which is all the more flat for coming after יסגו אחור. In Psa 70:4, after ויאמרים the לי, which cannot here (cf. on the contrary, Psa 35:21) be dispensed with, is wanting.
ויאמרו instead of יאמרו is unimportant. But since the divine name Jahve is now for once chosen side by side with Elohim, it certainly had a strong claim to be retained in Psa 70:5. Instead of תּשׁועתך we have ישׁועתך here; instead of עזרתי, here עזרי. And instead of אדני יחשׁב לי we have here אלהים חוּשׁה־לּי - the hope is turned into petition: make haste unto me, is an innovation in expression that is caused by the taking over of the לי.