Quick answer
What keeps a king in place is not force. The verse names loyal love and reliability as the actual load-bearing structure.
Read both lines
The word behind both "mercy" and "lovingkindness" is chesed, the covenant term, and the NKJV footnotes it in the second line so the reader can see the repetition. It is paired here with emet, truth or dependability, and that pairing is one Scripture normally uses of God. Applying it to a monarch says the throne is held up by the same qualities that hold up a covenant. The chapter has just described a king separating and threshing, so this verse arrives as the necessary other half. Power that only sifts does not last. What the second line says is that the king's own steadiness is the support, not his army.
Related: Proverbs 20:26 · Proverbs 16:12 · Proverbs 29:14 · Proverbs 3:3
New King James Version®, Copyright © 1982, Thomas Nelson. All rights reserved. Full copyright information. Scripture provided by API.Bible.
Go Deeper
If this verse has been used to put weight on you that it was never carrying, ask about it directly.