The KGB and its related intelligence services inside the USSR enjoyed an enormous degree of success during the cold war, with an alarming number of agents in important places, sometimes for decades. Many never got caught before the end of the cold war.
Although the USSR dissolved and most of their overseas assets were neutralized in one way or another throughout the 1990's (it's initial dissolution, the release of sensitive archive material, the Mitrokhin archive etc), the FSB, inheriting the resources and in many cases even personnel of the KGB, was certainly capable of rebuilding itself as a potent force. Having dirt on a POTUS would be a first, but I would not recommend underestimating Russian intelligence services. Even during times of major economic turmoil in the 20th century, the KGB significantly outperformed western intelligence services.