Short answers, pulled from the story.
Modern liberalism is the dominant version of liberalism in the United States since the mid-to-late 20th century. It is closest in meaning to social liberalism worldwide, combining cultural liberalism, progressivism, civil liberty and social equality with support for social justice and a mixed economy.
Modern liberalism took shape in the 20th century as a response to the Great Depression. Its roots reach back to Theodore Roosevelt's Square Deal and New Nationalism and Woodrow Wilson's New Freedom, and it developed further through Franklin D. Roosevelt's New Deal, Harry Truman's Fair Deal, John F. Kennedy's New Frontier and Lyndon Johnson's Great Society.
In Europe, liberalism usually means classical liberalism, a commitment to limited government and laissez-faire economics that corresponds more closely to what Americans call libertarianism. Historian Arthur M. Schlesinger Jr. wrote in 1956 that American liberalism 'has little in common with the word as used in the politics of any European country, save possibly Britain.'
Key figures include Franklin D. Roosevelt, Harry Truman, John F. Kennedy and Lyndon B. Johnson, whose New Deal, Fair Deal, New Frontier and Great Society programs defined modern liberal policy. Senators Hubert Humphrey and Paul Douglas and labor leader Walter Reuther led the mid-century liberal coalition in Congress and organized labor.
The Vietnam War split liberals into pro-war and anti-war factions, and the New Left attacked the movement from the left while conservatives attacked it from the right. Ronald Reagan's 1980 election, which followed Ted Kennedy's failed primary challenge to Jimmy Carter, marked the first non-Keynesian administration and the end of the liberal consensus.
The Northeast, the Great Lakes region, parts of the Southwest and the West Coast are the main liberal strongholds in the United States. Self-identified conservatives in Massachusetts are as low as 21%, and urban cores of large metropolitan areas lean more liberal and Democratic than surrounding rural areas.