Two factors that Schlesinger talked in his writing, one of them, is the process of appropriation, and another one is the war making power by the presidency. The process of appropriation took place in the twentieth century both in domestic and foreign affairs. The constitution allows a president to use certain power individually or with the help of three branches of the government. But in that time, presidency crossed the constitutional limits and became out of control. As the constitution tells the president to share the power with the congress. But in this point, president did not maintain the process of the constitution and took the individual decisions.
The war making power also made the presidency imperial. Because the constitution allows congress only to make the decision of initiate a war. Here, the president is allowed to maintain the foreign relationships and policies. A president could only response to the sudden attacks if congress is not present there. But there is no power given to the president to declare a war under any circumstances. But the United States became a great and powerful nation in the world and the presidency got more powerful to use them. This is how the imperial presidency took place and shifted the constitutional power from the congress to the presidents.