Pravda was and continues to be the mouthpiece of the Communist Party in the old Soviet Union, the remnants of which are looking for a way to reseize power today. Kerry has already shown that he is willing to try and tinker with other countries' internal politics (he proved this when he sent his sister down to Australia to try and throw John Howard out of office), and he hasn't even beeen elected yet! Is he giving aid and comfort to the commies in Russia, promising to help depose Putin and put them back in power if he wins?
I think Putin's endorsement made Kerry mad.
I don't think it's a good idea to make a megalomaniac mad.