The second mistake that has undone post-Soviet regimes in the past is official impunity — above all, brutality by law enforcement agencies. When the ruling party in Georgia lost parliamentary elections in 2012, the trigger was a viral video of torture by prison guards. In 2013 and 2014, the Ukrainian crowds that supported a “European” future for their country might easily have petered out but for popular outrage over a series of late-night police attacks on demonstrators. Last year’s abrupt overthrow of Armenia’s rulers, who had been in power for 20 years, reflected the same sort of anger, over the killing of protesters a decade earlier.

Now Putinism has again checked this impunity box. The planting of evidence by the security services against Ivan Golunov, a well-known investigative reporter, in June was so blatant and clumsily handled — and created such a storm of criticism — that the Kremlin ordered his release within days. The use of force to break up last Sunday’s election demonstrations, plus the apparent poisoning of the jailed opposition leader Alexei Navalny, risk creating similar outrage.

President Putin used to handle such problems more deftly. His is a soft dictatorship, and he has rarely risked his position with mass beatings and bloodshed, or too obvious fraud at the ballot box. The phony election results of 2011 were a clear exception, but the government dealt rather skillfully with the aftermath: Large protests were allowed to take place, and Mr. Putin cleverly appointed a well-known human rights advocate, Ella Pamfilova, to lead the Central Election Commission. (She has not disappointed him.)

So why might Mr. Putin and his helpers be losing their touch now? Why would they overreact to the danger of a small beachhead of liberal activists on the Moscow City Council? Ask knowledgeable Russians this question, and you are likely to hear about the politics of succession. Under Russia’s Constitution, the president cannot run again when his term expires in 2024, but even that distant prospect has rattled the elite. Polls show public confidence in him has declined. His party, United Russia, is so unpopular that it lost a number of gubernatorial elections last year, and many of its candidates in local races are choosing to run as independents. The economy continues to stagnate. In this environment, some analysts say, Russian officials at all levels are asking whether Mr. Putin can guarantee institutional and social stability — and their personal safety — as he once did. If he can’t, making sure elections look free and fair may begin to look like a luxury that the regime can no longer afford.

It’s not hard to see how Mr. Putin and those around him might also make the second mistake — brutality and impunity — that has brought down post-Soviet leaders elsewhere. For two decades the president has served as the ultimate protector of Russia’s so-called “power ministries,” treating cover-ups as part of his job description. Did two nitwits from military intelligence bungle their assignment to poison a Russian defector in Britain last year? No matter. The president had their back. Did associates of the Chechen leader Ramzan Kadyrov, Mr. Putin’s ally, murder the best-known leader of the opposition, Boris Nemtsov, right in front of the Kremlin in 2015? The president waves the matter aside. Mention any gross misuse of power by officials of the Russian military, security services, or police, and chances are Mr. Putin has publicly belittled its importance or exonerated those behind it.