Which development in the 20th century most reflects the increasing power of the president

The 21st Century dawned on a very different presidency than the one created at the end of the 1700s. Constitutional provisions limited the early presidency, although the personalities of the first three - George Washington, John Adams, and Thomas Jefferson - shaped it into a more influential position by the early 1800s. However, throughout the 1800s until the 1930s, Congress was the dominant branch of the national government. Then, throughout the rest of the 20th Century, the balance of power shifted dramatically, so that the executive branch currently has at least equal power to the legislative branch. How did this shift happen?