A Democrat was a member or supporter of the Democratic Party, a political party in the United States of America during the 20th century.
The President of the United States in 1947, Harry S. Truman, was a Democrat. After Truman refused to deal with Quark until he learned more about him, General Rex Denning said "that little piano-playing Democrat's not as dumb as he looks." (DS9: "Little Green Men")
Although not specifically referenced on Star Trek as members of the Democratic Party, other Democrats of note include Woodrow Wilson, Franklin D. Roosevelt, John F. Kennedy, Lyndon B. Johnson, Jimmy Carter, Bill Clinton, Hillary Rodham Clinton, Tip O'Neill, Sam Rayburn, and John Glenn.