It seems that Russian Ambassador Sergey Kislyak met with quite a few officials in the Donald Trump campaign before and after the election. Who knows what was discussed but the real question is whether or not Kislyak wore a wire. My guess is that he did and therefore the Russians have transcripts of exactly what was said. Given that Attorney General Sessions has already testified under oath and will soon do so again and the other Trumpista's will soon be facing subpoenas, there certainly is the potential for blackmail.
Further although Kislyak is probably not an officer in the SVR (the Russian foreign security intelligence service) his "Rezidentura" (station chief) reports to him as well as Moscow Central and as is typical for most governments the ambassador and his/her security chief work hand in glove together.
Thus President Trump has more to worry about than the alleged without evidence Obama wiretap; his friends in Russia could very well have a real hold on him and his administration.