"Biden has a reputation as a populist, and he’s a fairly liberal Democrat, but in the 110th Congress Clinton was the 11th most liberal senator, while Biden ranked 33rd (Obama was 23rd.)"
--
Joan Walsh, Salon
Unlike European designations (Tory, Conservative, Labour), the United States has not held to rigid sub-categories in its political labels. The American tradition has been one of Yankee pragmatism. Compromise and flexibility are valued, independent thinking rather than a strict code. This accounts for the slipperiness of some political labels beyond the operative labels--Democrat and Republican.
*****
Addendum: Speaking of political designations, the 2016 field of candidates for the nomination includes a new one. Bernie Sanders is called a Socialist, to which he always replies:
"I am a Democratic Socialist."
The distinction may have as its origin a highly regarded man named Norman Thomas, an American Presbyterian minister who achieved fame in the 1930s and 40s as a socialist, orator, and six-time presidential candidate for the Socialist Party of America.
Thomas called himself a Democratic Socialist,drawing a bright line between his beliefs in participatory democracy as opposed to authoritarian or totalitarian communism.
Democratic Socialists like Norman Thomas do believe in capitalism and private enterprise, and want as many people as possible to benefit from private enterprise; but not at the expense of the middle and working class.
Thomas was born in 1884 in Marion, OH and died in 1968 in Cold Spring Harbor, NY.