The Associated Press Stylebook (2007) devotes a very useful entry to "political parties and philosophies." This entry indicates in some detail how newspapers that follow AP style should handle terns such as "social democracy" when they appear in various contexts. Here is the entry:
political parties and philosophies Capitalize both the name of the party and the word party if it is customarily used as part of the organization's proper name: the Democratic Party, the Republican Party.
Capitalize Communist, Conservative, Democrat, Liberal, Republican, Socialist, etc., when they refer to a specific party or its members. Lowercase these words when they refer to political philosophies (see examples below).
Lowercase the name of a philosophy in noun and adjective forms unless it is the derivative of a proper name: communism, communist, fascism, fascist. But Marxism, Marxist; Nazism, Nazi.
EXAMPLES: John Adams was a Federalist, but a man who subscribed to his philosophy today would be described as a federalist. The liberal Republican senator and his Conservative Party colleague said they believe that democracy and communism are incompatible. The Communist said he is basically a socialist who has reservations about Marxism.
So in your two examples—assuming that they abide by AP's style guidelines—the first ("[I]n a vain effort to combat Social Democracy, he seriously interfered with the liberty of public meeting..."), as you surmise, must refer to a particular political party with the formal name "Social Democracy Party"; and the second ("[T]he author observes that the US already has elements of social democracy") must refer to "social democracy" as a political philosophy independent of any particular party that may bear that name.