The correct answer to this open question is the following.
In this case, a President of the United States would want to become a dictator or to have so much that could dominate the other two branches of the federal government, the American citizens hold the key to depose him. That is what the concept of people's sovereignty dictates in the Constitution.
So if there is a case when the President of the United States attempts to undermine democratic norms and tries to turn into an authoritarian ruler or dictator who consolidated political power and could make virtually all political decisions on their own, there are the legislative branch and the judicial to exert their powers and try to put a stop to that president's aspirations.
In the specific case of the legislative branch, the US Congress, it is the citizens the ones who vote and elect their representatives and have all the right to demand positive changes and actions. Under the concept of popular sovereignty, the American people can demand Congress to impeach the US President and to follow proper mechanisms under the Constitution to make a change.
The US had just had a similar situation with President Trump, not exactly a dictator because it was never the case. But a President that notoriously tried to exert his power over the other two branches. He impacted US politics in many ways, but Congress, specifically, the lower chamber of the House of Representatives, served as the "checks and balances" branch that always tried to put a stop on him. Indeed, the House started the impeachment process.