MOSCOW — The most cutting blow lately against Russian President Vladimir Putin was not President Biden's recent "killer" ­epithet but the taunt from jailed Kremlin critic Alexei Navalny calling him "grandpa in his ­bunker."

Shut off from the public and surrounded by security services, Putin has been largely holed up in his country retreat, protected by a special disinfection chamber for visitors. Navalny’s repeated use of the barb stung because it flipped Putin’s image: no longer a Russian bear but a cowering senior.

Cue a state propaganda blitz with 68-year-old Putin portrayed as a dynamic leader with years ahead of him and 78-year-old Biden as a doddering has-been.

Parliament member Pyotr Tolstoy said Biden had “political dementia.” State TV anchor Dmitry Kiselyov called Biden “the American patient,” noting that “at some point you have to face up to the inevitable” of age catching up. Kiselyov gleefully aired footage this month of Biden stumbling on the steps to Air Force One with the on-screen banner “Gone With the Wind.”

Biden triggered a clamor of official outrage in Russia after he agreed in a March 17 ABC News interview that Putin was “a killer.” But some Russian analysts argued that Biden had given Putin exactly what he needed, with his United Russia party sinking in popularity and elections ahead this fall.

Biden made Putin seem powerful, facing down an external enemy, instead of trying to cocoon himself from the pandemic. (Putin received his first coronavirus vaccine dose Tuesday, the Kremlin said.)

“In terms of domestic politics, [Biden’s comment] helped Putin, because he was running out of topics,” said political analyst Konstantin Gaaze of the Carnegie Moscow Center. “Anti-Americanism is something that can be heated up. The basic mode is anti-Western, and it’s not the first time, or last time, it is used.”

An editorial in the business paper Kommersant predicted that “thanks to Biden” Putin’s United Russia might win every seat in the lower house of parliament.

“Joe Biden seems to have launched a major theme for uniting society around Vladimir Putin ahead of the September Duma elections,” it said. “The timing of Biden’s help couldn’t have been better. The image of an enemy, exploited since 2014, had begun to fade.”

Putin, who has never debated a domestic political opponent in his life, challenged Biden to a live debate March 18 (as if live debates between world leaders are a thing). But then Putin headed off to the Siberian taiga for the weekend as he begins to clamber out of his reclusion.

The Kremlin is highly sensitive about how Putin looks. It fiercely denied a December report by Russian investigative outlet Proyekt that the president was actually living in Sochi on the Black Sea and had a replica of his Novo-Ogaryovo office outside Moscow built in his Sochi residence to hide that fact. Proyekt relied on flight records and Kremlin sources.

In January, Putin was put on the defensive by Navalny’s bombshell YouTube video of a vast palace on the Black Sea titled “Putin’s Palace: History of the World’s Largest Bribe,” viewed more than 115 million times. The Kremlin denied the palace was built for Putin, and the president’s old friend and judo partner, billionaire Arkady Rotenberg, later claimed ownership.

Meanwhile, Russia was hit with new sanctions from the United States and the European Union over the Russian authorities’ poisoning and subsequent detention of Navalny, and additional sanctions are possible. Russia denies any role in the poisoning.

Navalny’s jailing in January on his return to Russia — and a raft of draconian laws to crush dissent — sparked protests in more than 100 Russian cities and ignited anger among young Russians, many of whom see Putin as staid, repressive and distinctly uncool.

But Putin has recently burst back into the public eye. He drew a packed, cheering crowd to Moscow’s Luzhniki Stadium to mark the anniversary of Russia’s 2014 annexation of Crimea from Ukraine.

Putin then took Kremlin pool journalists to Tuva in Siberia, where he drove through a snowy forest in a hulking all-terrain vehicle and sipped tea at a rustic outdoor table. The event was planned well before Biden’s “killer” comment, but it delivered Putin the chance to pose as a nature-loving “muzhik,” or real man.

Putin on Thursday promised a slew of new trips, following his recent secretive vaccination.

“He was telling people he is in good health and he’s still close to Mother Nature. There was a strong message to this electorate that he’s in the taiga doing manly things,” Gaaze said.

In Siberia, Putin wore a quaint sheepskin jacket and pants, as did Defense Minister Sergei Shoigu. Headlines in pro-Kremlin media were even more over-the-top than usual: “The rarest shots! The Russian president has never been seen like this!” one outlet gushed. One image showed him jumping out of the driver’s seat.

“Just don’t show this to Biden,” tweeted Kremlin pool journalist Dmitry Smirnov.

Independent pollster the Levada Center reported this month that support for Putin’s party dropped to an eight-year low, with just 27 percent willing to vote for it if elections were to be held March 14.

Opposition to Putin staying in power beyond 2024, when his current term expires, reached an all-time high of 41 percent in a February Levada poll, compared with 48 percent who would like him to stay in power.

In a meeting last month with newspaper editors, Putin spelled out his ideology, boasting of Russia’s patriotism and its unique “genetic code” that distinguishes it from the West.

Russia, he said, is a young, vital nation “on the move,” compared with other aging societies.

But Gaaze said this ideology has limited appeal and that Putin’s era of political invulnerability is showing signs of fatigue.

“He’s basically a Soviet man who cannot remake the Soviet Union, but he is trying his best to do so,” he said. “He thinks he is the only man up there who understands [ordinary Russians’] mind-set.”

For many people, Gaaze said, Putin’s talk of Russian dignity and complaints of Western insults to Russia’s Sputnik vaccine do not resonate deeply.

“It’s not the first or third or even the fifth thought of ordinary Russian citizens,” he said.