Radak: This is like Lehariv. Also "Ladin" is like Lehadin. If it were a noun, there would be a Sheva under the Lamed. Do not think that Hashem will not punish you for the evils that you do. At all times He stands to judge all the Nochrim, and all the more so His nation Yisrael. His Hashgachah is greater on them, for they are closer to Him than other nations.

Malbim: When Hashem wants to judge His nation for their evil deeds, and sees that the primary blame is on the Zekenim and the judges who corrupted the nation, He will judge and quarrel also with them. Refer to 3:13:2:2.

Rashi (13, 14): Omed is an expression of delaying. The Midrash explains that this refers to joining the nations. The simple meaning refers to Yisrael; "Amim" are the Shevatim.

Malbim: Hashem is depicted as a judge ("Omed Ladin") and litigant ("Nitzav Lariv.")