MOSCOW — The flowers are constantly replenished on the Bolshoy Moskvoretsky bridge near the Kremlin's walls,  at the spot where liberal opposition leader Boris Nemtsov was gunned down in February 2015. A volunteer cadre of Nemtsov's admirers regularly resupplies the blooms and tries to guard the memorial from destruction by thugs.

A visit to that bridge is a potent reminder of the risk of publicly opposing the Kremlin.

As Vladimir Putin prepares for his fourth term as president – a post that many Russians doubt he will willingly leave –  the obstacles that face any viable Russian opposition leader seem overwhelming. Even apart from the danger.

Which is why I set out to meet with the dynamic 41-year-old Alexei Navalny, the only opposition leader with nearly 50 percent national name recognition and a countrywide following — even though his name is rarely if ever uttered on state-controlled TV.

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Navalny was banned from competing in the recent presidential election after trumped-up embezzlement charges conveniently disqualified him.

"After they decided not to give me permission to run, we understood the basic decision of Putin is to be a lifetime president and die in the Kremlin like a tsar," Navalny told me.  This unconventional politician  and his youthful team are now trying to devise a strategy that will enable them to keep pushing back against those terrifying odds.

What makes Navalny's movement so fascinating is the way he built his following and his reputation. I met him in his sleek, spare office, in the Omega Plaza business center far from the city center, staffed by well-dressed young people who typify his legions of followers.

A lawyer by training, he has built his national base by an indefatigable campaign against Kremlin corruption.  With no access to state-controlled media, he has waged that campaign via YouTube, the Telegram messaging service, and other social media.

Using internet tools and 20,000 young volunteers, he won more than 27 percent of the vote in the 2013 Moscow mayoral campaign despite predictions he'd get only between 5 percent and 6 percent.  He also amassed  a war chest of online contributions. "We copied Obama's method of fund-raising," he told me.

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No wonder he was banned from running against Putin in 2018.

In a country where a coterie of Kremlin-favored oligarchs sock away billions, Navalny's campaign has made its mark. To get the flavor, you need only watch the YouTube video his Anti-Corruption Foundation put out in March 2017 (with English subtitles) documenting the alleged ill-gotten assets collected by now-Prime Minister Dmitry Medvedev, including mansions, yachts, and even a vineyard.

This video has collected 27 million views. It sparked nationwide demonstrations in late March that startled the Kremlin, because they drew tens of thousands of demonstrators in cities, towns, and villages across the country.

"Since March," Navalny says,  "our main tool became YouTube because it attracts more younger people."  He says the nationwide demos drew "blue-collar youth, not hipsters, youth who see no future. Even a university graduate, the best salary he can hope for is $1,000 a month. No future at all."

But how does he, and they, move forward after an election in which Putin claims to have won 76 percent of the vote with a 65 percent turnout? Navalny claims it was only 52 percent turnout, as a consequence of the boycott his movement promoted. But still.

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"All authoritarian regimes have such elections, but our main goal is to impress people in the opposition that this result means nothing," says Navalny. "These percentages are another leverage to persuade people everything is fruitless, that resistance is impossible. "

"Our goal is to stay as a mass movement  linked by YouTube, Telegram,  and instant messenger," he says. "We don't have a license to be a political party, we don't have deputies [in the Duma], but we are the biggest movement in Russia," he says.  "No village without our supporters."

Yet, despite the bravado, this would-be politician understands the odds.

The authorities now ban his movement's outdoor rallies and have arrested thousands of his followers. Critics question whether his rallies expose young people to undue risk.

Kremlin trolls call him a "fascist" and a "traitor."  A year ago, a thug threw caustic fluid on his face and nearly blinded him in one eye; he has mostly recovered but the culprit, whose identity is known, has not been punished. Navalny's brother has been jailed for more than three years on more trumped-up charges. At any moment Navalny himself could be thrown in prison.

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Then there is the fact that Russia just tried to shut down the Telegram messaging service. The effort was an embarrassing failure, but Navalny believes there will be future Kremlin efforts to control the internet.  He insists, however, that they won't be able to emulate Chinese social-media controls.

Yet he won't give up.

"I'm a democrat and want rule of law and for Russia to be part of Europe,"  Navalny says.  If he's given a chance to run again, he thinks he and his movement can eventually win power. "When you participate in elections, people see who you are.

"When you talk about corruption of Putin or the [Moscow] mayor, these are things they never heard before. Otherwise they think you are an American spy."

Indeed, relentless negative propaganda about Navalny on state media has had an effect, if one can believe the polls.

Of course, having visited the  Nemtsov memorial,  I couldn't refrain from asking whether he feared he might be killed.

"They [the Kremlin] missed a chance when I was less popular," he replied with aplomb. " Now they are afraid they will provoke rallies or worse. Right now the cost is higher than they want. "

But he adds with grim humor:  "Maybe they are still saving this tool. After the [World Cup] championship games in June, who knows what may happen."

The interview finished, as Navalny prepared for his next YouTube live talk on how Russian oligarchs bribe officials. And he has called for nationwide protests two days before Putin's May 7 inauguration.  Amazingly, his anticorruption movement goes on.