A week before Russia’s presidential elections—to be held on Sunday, March 4—the country’s former president, Prime Minister Vladimir Putin, drove home a forceful message about “betrayal.” In front of a stadium packed with bussed-in supporters, he gave a passionate oration: “Do not run to the left, to the side, and do not be unfaithful to the motherland.” Meanwhile, outside, military vehicles lined up along the main streets in Moscow and St. Petersburg, ready to detain the opposition activists who have been an increasing menace to Putin’s uncontested grip on power.

Putin has some cause to worry. True, the polls show that he has reversed his sudden drop in popularity—which began to plummet last December as people turned out to protest against supposed election fixing—and analysts expect him to win by 66 percent in the first round of voting next Sunday. Still, today’s political climate feels much more like a spring—certainly compared to Putin’s landslide victories in 2000 and 2004. Protests have roiled even the most indifferent Russian regions, and “Russia Without Putin” movements are a common sight.

Perhaps the biggest threat to Putin in this year’s election is the opposition candidate, 46-year-old billionaire Mikhail Prokhorov. For the first time, Russians have a real alternative to Putin, and he’s promising something like real reform. Indeed, Prokhorov—Russia’s third-richest man—is showing all the signs that he might one day grow into a real political force.

The billionaire emerged onto Russia’s political scene last summer, as the leader of the liberal Right Cause party. Opposition leaders criticized him for being a Kremlin lackey. But when the project was sabotaged by the Kremlin for showing worrying signs of a true opposition agenda, many Russians started to pay attention to the tall, laconic oligarch (who owns the New Jersey Nets basketball team).

Some critics still accuse Prokhorov of coordinating his activities with Putin. “A couple of months ago, Putin and Prokhorov spoke on the phone,” claims independent analyst Stanislav Belkovsky, “and agreed that Prokhorov would run as an opposition candidate.” And Prokhorov has been careful not to say anything that would offend Putin personally. But the billionaire’s presidential program promises to revoke many of the structures put in place by Putin, including the six-year-long presidential term. Prokhorov is also in favor of letting media, Parliament, and the economy become independent of the state. And he has been a prominent figure at rallies for free elections.

Prokhorov’s commitment to liberal change comes as no surprise to anyone who has known his family, and particularly his older sister, Irina Prokhorova, who is often seen at Prokhorov’s side during protests. The siblings are close, bonded in tragedy—they lost their parents when they were young—and now live on the same estate outside Moscow. For years Prokhorova has managed her younger brother’s charitable foundation and directs a publishing house, New Literary Review. Prokhorova has been a forceful advocate of liberal ideas long before her bachelor brother voiced them. During the campaign, she has acted as Prokhorov’s surrogate in debates (a common practice now in Russia, as Putin refuses to participate in debates and offers up his substitutes to debate in his stead), giving speeches on social issues and otherwise stepping into the role of Russia’s first sister.

Prokhorova—tall, with a sporty posture, a chic bob and a modest fashion sense, brings a gravitas to her brother, who is famous for stocking his parties with teen beauties and who was once arrested by French police in the Alps for alleged involvement with a prostitution ring. Prokhorova, by contrast, is a family woman with an academic air. While Prokhorov has charmed voters with his rapping skills in boyish campaign advertisements, Prokhorova goes on the attack to defend her brother on radio marathons. Many claim she is the more charismatic sibling: “I would rather vote for her,” says Russian opposition leader Boris Nemtsov.

During her debates, Prokhorova has blamed Putin for “pushing Russian culture to the verge of crisis” during his 12-year rule as president and prime minister. A Prokhorov administration, she tells The Daily Beast, would measure Russia’s greatness by its cultural growth—particularly by investment in the Internet, universities, village schools, and provincial libraries.

Prokhorova has been a longtime advocate of Russian culture, starting her publishing house in 1992 to “raise a new generation of proud intellectuals.” While she was running in literary circles—and visiting America for the first time in 1994—her brother was busy building his $18 billion fortune off of Norilsk Nickel. Now the world’s largest nickel and palladium company, Prokhorov bought it for a pittance from a cash-strapped Kremlin in 1995, in a scheme known as loans-for-shares that still stirs controversy in Russia today. The privatization of the Norilsk Nickel mine in Siberia wouldn’t have been possible without the support of Prokhorov’s close friend at the time, then–vice prime minister Vladimir Potanin. In 2008, Prokhorov sold his shares in Norilsk to another Russian billionaire, Oleg Deripaska. But many Russians still feel that the oligarchs of the 1990s—including Prokhorov—were thieves, and it remains Putin’s favorite myth that he alone brought the oligarchs to heel. “Being an oligarch is Prokhorov’s weakest spot in the eyes of Russians,” says opposition leader Nemtsov.

Another potential stumbling stone: Prokhorov’s personal life and his playboy image. At the latest televised debate, between Prokhorova and film director Nikita Mikhalkov, a Putin supporter, Mikhalkov attacked the billionaire for not being religious, married, or a father. “He would not really understand families or children’s issues,” he said. “Enough, I got you,” Prokhorova interrupted softly, her cheeks flaming. “Maybe he has not met his love.”

“It is not my brother’s personal life that is important, but for him to become a leader for successful young Russians.”

Still, the attacks on Prokhorov by Putin’s supporters signal that the billionaire has gone from being a tame stooge to a real and growing danger to the Kremlin. Putin will surely win this Sunday, but Russia’s elite is already busy maneuvering for the endgame of his rule. And right now, Prokhorov seems the man most likely to be there, waiting to take the reins of power—either as a friendly successor to Putin, or a hostile one.