Presidential power has grown in proportion to the federal government; as the federal government assumed greater authority over many aspects of citizens' lives, the nation placed the responsibility for these functions in the hands of an executive, as opposed to a Congress composed of hundreds of people. The founders did not envision the modern extent of presidential power. These changes arose out of historical exigencies and decisive personalities.
The Constitution devotes relatively little attention to the executive branch. The founders intended Congress to be the dominant branch of government. The first presidents, Washington, Adams and Jefferson, were extremely renowned statesmen and national leaders. Their personalities contributed as much as their policies to the growth of the presidency. Thomas Jefferson worked out the Louisiana Purchase in 1803 despite the lack of constitutional authority to do so.
Later, Andrew Jackson continued the trend. He made intense use of the veto and asserted national power by striking down South Carolina's nullification of a federal tariff law. During the Civil War, Abraham Lincoln assumed unprecedented executive power to meet the needs of the conflict. A sequence of laid-back presidents followed until Theodore Roosevelt and Woodrow Wilson, the progressive presidents, came to office. The former used the office to initiate widespread social reforms, while the latter expanded the president's role in international affairs.
After Wilson, Congress again took precedence over the presidency. However, the Great Depression convinced Americans to give the federal government a greater role in the economy. Thus, Franklin Roosevelt amassed great power. World War II and the Cold War increased the president's role, and presidential power has continued to grow ever since.