A few years ago, two Russian friends made a pledge to play more sports and stay in shape. Mikhail Prokhorov, a billionaire oligarch, and Boris Nemtsov, an aggressive opposition leader, shared more or less the same liberal views, and their decision to stay fit and healthy was driven by a political hunger: they wanted to live to see a time when Prime Minister Vladimir Putin’s authority began, finally, to decline. But for all the two men’s shared tastes—for democracy, for sports, for beautiful women—they could never agree on one crucial thing: whether they should compromise with their archenemy Putin.

On Monday they finally clashed. Prokhorov, who owns the New Jersey Nets, announced that he would run for president against Putin, an act that means he recognizes the current prime minister as a legitimate candidate. Nemtsov and other opposition leaders, meanwhile, are calling for Russians to take to the streets next week and demand Putin’s resignation.

Flirting with a crowd of journalists this afternoon, a playful smile on his lips, Prokhorov said he had made “the most serious decision” of his life. The oligarch—chosen by the Kremlin in June to lead the newly created pseudoliberal Right Cause party before being ousted in September—would become a candidate in the presidential election in March. If he failed to win, he said, he would create a new party that would support the Russian middle class.

But what about the revolution Nemtsov and the other opposition leaders say they’re planning? “I am against bloody revolutions, I am much more in favor of evolutions,” Prokhorov told reporters. Independent candidates have often been rejected by Russia’s election commission when trying to register, so journalists asked the tycoon repeatedly whether his decision to run had been approved by Putin or by the Kremlin’s administrator for political affairs, Vladislav Surkov. Prokhorov denied he had met with either of them.

“He is lying to you!” Nemtsov exclaimed in an interview Monday. “Oligarchs who do not make deals with Putin go to jail in Russia. The crucial difference between Prokhorov and us, the independent opposition, is that we want to relieve Russia from Putin’s regime. Prokhorov wants to improve his career.”

Nemtsov seemed furious; he said that he was supposed to meet with Prokhorov a month ago, but he’d been too busy preparing and leading the "Snow Revolution” protests. His old friend’s presidential run is designed to undermine those protests, he said. “Prokhorov’s role in the game would be to get some of the liberal electorate off the streets. But he is walking on thin ice. Those who call Putin a thief will immediately recognize Prokhorov’s decision as a betrayal.”

For the last five years, Nemtsov, a former deputy prime minister, has been leading anti-Putin street protests, but they never attracted more than a few hundred people. Prokhorov, who made his $18 billion fortune mainly in precious metals, didn’t attend any of those protests, comfortably existing within the system as co-chairman of the pro-Kremlin Union of Industrialists and Entrepreneurs. Always cautious about how he characterized Putin in public, Prokhorov called Putin Russia’s only popular political figure in a recent lecture to students at Moscow International University.

But Prokhorov and Nemtsov had privately formed an alliance, Nemtsov said Monday. “We decided we would pump up our muscles, dry up like two butterflies, and wait for the right moment to fly off the wall and begin to act,” he said.

Nemtsov joked about their plan, which he said they formulated after Nemtsov abandoned his own presidential campaign in 2007. Thereafter, Putin’s youth activists attacked both Nemtsov and Prokhorov. But if Nemtsov was prosecuted for his opposition activities, Prokhorov was attacked mostly for his lifestyle. Pro-Putin activists poured a pail of chocolate over Nemtsov during one of his public speeches and called him the “people’s enemy,” a marginal politician plotting a revolution using American money. Outside a conference for Prokhorov’s Right Cause party, Putin activists acted out a parody of Prokhorov driving a carriage pulled by long-legged girls, mocking the oligarch for his reputation as a womanizer.

On Dec. 31, 2010, Nemtsov was arrested and later sentenced to 15 days in jail for taking part in an anti-Putin rally. And Prokhorov was criticized by state-run newspapers for his enthusiasm in peopling his parties with breathtakingly beautiful models.

In September Prokhorov appeared to hit back. He publicly blamed Putin’s “puppet master” Surkov for sabotaging the electoral prospects of the Right Cause party and ousting him as leader.

“On that day I said that a new opposition leader was born, as not many dared to attack untouchable Surkov before Mikhail Prokhorov,” Duma deputy Gennady Gudkov recalls. “And so I’m more surprised today that after being humiliated by the Kremlin in front of the entire country, Prokhorov does not aggressively seek revenge but comes back to help the Kremlin legitimize the presidential elections.”