Jamieson, Fausset and Brown on Jeremiah 21:12

12. house of David--the royal family and all in office about the king. He calls them so, because it was the greater disgrace that they had so degenerated from the piety of their forefather, David; and to repress their glorying in their descent from him, as if they were therefore inviolable; but God will not spare them as apostates.

in the morning--alluding to the time of dispensing justice (Job 24:17; Psa 101:8); but the sense is mainly proverbial, for "with promptness" (Psa 90:14; 143:8). Maurer translates, "every morning."

lest my fury ... like fire--Already it was kindled, and the decree of God gone forth against the city (Jr 21:4, 5), but the king and his house may yet be preserved by repentance and reformation. God urges to righteousness, not as if they can thereby escape punishment wholly, but as the condition of a mitigation of it.