Political Cartoon by Dave Granlund of Massachusetts and published on 31 October 2010.
The cartoonist is illustrating how the Republicans, represented by the elephant’s foot, are predicted to crush the Democrats in the midterm elections. The donkey states that the GOP (Grand Old Party, i.e. the Republicans) has a leg up on the midterms. If you have a leg up on somebody, this means that you have the advantage. For an example, if you have more experience than other candidates for a particular job, you probably have a leg up on the other applicants. This expression is American.