How has the role of the Presidency changed from 1789 to the present?
The role of the president in the early days of the country was quite limited. In many ways, the Constitution gave a great deal more power to the Congress, and even the president's means of checking the Congress, such as the veto, were rarely used. For example, Washington used the veto twice, while Adams and Jefferson did not use this power at all. There were some presidents, such as Jackson and Lincoln, who exercised more power, but, by the late 1800s, the president (and national government in general) had a limited role in people's lives.
However, presidents in the twentieth century began to vastly expand the role of the presidency, particularly through advocating a progressive agenda. Teddy Roosevelt's Square Deal, for example, involved busting monopolies and setting aside public lands for conservation. Franklin Roosevelt, who used the veto 635 times, initiated the New Deal to bring the country out of the Great Depression. He started entitlement programs such as Social Security that still exist today and vastly enlarged the role of the federal government in people's lives. Other presidents in the twentieth and twenty-first centuries expanded entitlement programs; for example, Lyndon B. Johnson started Medicare and Medicaid, and Barack Obama started national health insurance with the Affordable Care Act.
In addition, many presidents in the twentieth century conducted foreign wars without congressional approval. While Congress declared war during the War of 1812, the Mexican-American War, Spanish-American War, World War I, and World War II, Truman committed American troops to Korea under the United Nations without congressional approval. Kennedy sent advisors to Vietnam without the approval of Congress, and Lyndon Johnson used the Gulf of Tonkin Resolution (which was not a declaration of war) to conduct a full-scale war in Vietnam. Nixon continued the war in Vietnam. Presidents became increasingly willing to conduct wars without congressional approval, leading to the War Powers Resolution of 1973 to limit the president's ability to conduct wars without congressional approval. The resolution showed that the powers of the president had increased in the twentieth century.
The role of the presidency has changed in very major ways since 1789. In the originial Constitution, the president was seen as very much the junior partner to the legislative branch. This can be seen in the fact that early presidents believed that what they were supposed to do was to simply carry out the laws passed by Congress.
Over the years, partly because of the expanding role of the government, the role of the presidency has exploded. We now see the President as the one leader of the nation who is supposed to make sure that everything is okay. If the economy does badly, we blame the President. If our military does well in a given operation we give credit to the President. The President is now seen as the person who is supposed to dominate and set the national agenda and who is responsible for the overall well-being of the country. This is far beyond what the Founders expected the presidency to be.