Even if passed by two-thirds of both houses of congress a constitutional amendment still must be

After being passed by both houses of Congress, a constitutional amendment must still be passed by 3/4ths of the states. After it is passed by Congress and the states, it still must be approved by the US President. This is because the president has the power to veto any law passed by Congress. If the president vetoes (stops it from becoming a law) the amendment, the veto can be overrode by another vote in Congress.