Policemen block supporters of Russian opposition leader and anti-corruption blogger Alexei Navalny during a rally in protest against court verdict at Manezhnaya Square in Moscow December 30, 2014. (Tatyana Makeyeva/Reuters)

VLADIMIR PUTIN claims to have the overwhelming support of Russians after a year in which he launched an invasion of Ukraine, annexed part of it and positioned his regime as an autocratic and homophobic counter to the democratic West. If that’s the case, why does Mr. Putin appear to be so terrified of Alexei Navalny?

The anti-corruption campaigner, who has been under house arrest for months, was scheduled to be sentenced Jan. 15 in a trumped-up criminal case. But as tens of thousands of Russians signed up on social media to attend a pro-Navalny demonstration that day, authorities abruptly moved up the court date to Tuesday. In a further tactical maneuver, a judge handed Mr. Navalny a suspended sentence while ordering 3½ years of imprisonment for his younger brother, who is not involved in politics.

The “dirty trick,” as Mr. Navalny called it, didn’t entirely work. A crowd of several thousand chanting protesters still turned up at a square near the Kremlin on Tuesday night, despite police barricades and bitter cold.“You won’t be able to jail us all,” the protesters shouted.

That slogan may well come to haunt Mr. Putin in 2015. Russia’s central bank is predicting a sharp contraction of the economy in the coming months due to the plunging price of oil and Western sanctions. Holding on to Crimea, the isolated Ukrainian peninsula he seized, and preventing puppet “rebel” movements elsewhere in Ukraine from being overrun will cost the Russian ruler dearly, both in rubles and, likely, in Russian lives.

Mr. Putin’s strategy for overcoming these challenges will likely be the same as it has been since 2012, when Mr. Navalny helped to lead mass demonstrations against him: escalate domestic repression as well as military adventures in Ukraine and elsewhere, while bombarding the country with nationalistic propaganda. The cost of foreign adventures is escalating, however, and Russia’s oligarchs will be pressing the Kremlin to back off enough from Ukraine to obtain a weakening of sanctions.

Thanks in part to his media monopoly, Mr. Putin does register well in opinion polling. But Russian dissidents say they believe the support is thin and unlikely to weather either a plummeting economy or substantial military casualties. Until now, protests in Moscow have been easily contained by the regime’s riot police. Yet if Mr. Putin were unworried about that opposition, Mr. Navalny’s case would not have been handled with such panicked improvisation.

Whether resistance to the Putin regime grows will depend to a large extent on U.S. and European Union policy. European sanctions start to come up for review in March; maintaining them in the likely absence of a Russian withdrawal from Ukraine will require determined leadership from President Obama and German Chancellor Angela Merkel.

Support for Russian dissent also matters. As so often in human rights cases, the Obama administration’s reaction to Mr. Navalny’s sentencing was mild: The State Department said it was “troubled.” On Monday, however, the administration sanctioned four Russian officials under the Magnitsky human rights law, including two senior officials in Chechnya involved in the arrest and torture of a leading critic. Such actions can hearten those fighting for human rights in Russia. The judge and prosecutors involved in the prosecution of Mr. Navalny would be good candidates for the next round of sanctions.