The first quote is from Bohumir Smeral, one of the founders of the KSC (the party) in 1921. It reads: "We are not simply a political party. We are the vanguard of a new existence". The second quote is from Gustav Husak, Czechoslovak Communist leader in 1976 when the stamp was issued. It's less interesting and simply babbles on about the leading role of the party within the working class, its historical mission, etc.
The leafs in the background are presumably linden leafs, also seen in the flag of the Czechoslovak president, another office occupied by "comrade" Husak at the time.
Sometimes my knowledge of obscure stuff is downright scary!