You can tell the ruling party is in trouble when one of its candidates loses to his own office cleaner during municipal elections...
This opinion piece in the Washington Post by Russian democratic activist Vladimir Kara-murza points to the ultimate weakness in Putin's Russia.  It isn't just about designating the opposition as foreign agents.  It is about a host of measures calculated to strangle the opposition so that there is no opportunity for dissent and no effective constraint on Putin's power.
But as Putin's stranglehold on the political process grows, the risk to Russia's stability increases.  With no outlet to express grievances on a host of growing social and economic problems in Russia, the moment when the people find their voice is likely to be explosive (as it has been in Belarus).  As Kara-Murza writes, "If society is unable to express its opposition through the ballot box, it will find another way."