It would be the Supreme Court's duty and function to declare laws passed by Congress as unconstitutional. The executive branch would have to enforce the law enacted by Congress until it was legally challenged and adjudicated by the Supreme Court. In response to the other question in terms of Constitutional succession, the initial Act of 1792 provided for the presidential succession starting with the third in line being the President of the Senate, leader of the upper chamber, followed by the Speaker of the House, the leader of the lower chamber. This act was further amended in 1886 with new legislation that amended the old law to add the President's cabinet members into the line of succession starting with the Secretary of State. After several other legislative changes and a Presidential assassination in 1963, the final word on the Presidential line of succession was finalized with the passage of the 25th Amendment to the U.S. Constitution in 1967. I am surprised that the Congress and the Supreme Court did not clarify the line of succession earlier especially during Lincoln's Presidency with the numerous attempts to either kidnap or assassinate him or his cabinet members by the Confederate government and their sympathizers. David.