The antonyms of "undermine" are "strengthen", "reinforce", and "support".
"Strengthen" means to make something stronger or more powerful. For example, "Regular exercise can strengthen your muscles." When you strengthen something, you are adding to its stability and durability, the opposite of undermining it.
"Reinforce" also implies adding strength or support. It often suggests making something more resistant or firm. For instance, "The army reinforced the border defenses." Reinforcing an object or idea gives it more solidity and opposes the act of undermining.
"Support" means to bear the weight of something or give assistance. "We should support local businesses." By providing support, you are helping to maintain and build up rather than undermining.
In summary, while "undermine" weakens or damages, its antonyms "strengthen", "reinforce", and "support" work to enhance and uphold.