Why is the relation between the Congress and the President considered as the weak link in the American Constitution?

The relation between the president and the US Congress is considered a weak link because the two arms of government do not depend too much on each other especially when the president and the majority of the Congress member are from different political party. The office of the president is the head of executive arm of the government and is perceived and constitutionally strengthened to do some things as the head of the nation.

The US Congress is the legislative arm of the federal government and is divided into two, the Senate and the house of representatives. As big as the power of the president might look to you, the Congress always has their way. This is because the constitution has been made in such a way that it limits the power of a president so as to curb its excessive abuse.

There are some things a president cannot single-handedly do and that's because most of things he would do will need appropriate constitutional backup.