Proper noun. The 42nd President of the United States, William Jefferson Clinton. This nickname speaks to Clinton's continuing political clout, reputation for deftness and skill as a politician, and vaunted ability to campaign for Democratic candidates. If you can't run with The Big Dog, you need to stay on the porch, motherfucker!
"Down in the polls, Lieberman tried to call in The Big Dog to rescue his moribund campaign against Lamont. Problem is, The Big Dog only supports Democrats."