(Adds details on Right Cause's platform, analyst comment, political background)
* Reformists out of political mainstream under Putin
* Rare return of billionaire to politics
By Maria Tsvetkova
MOSCOW, June 25 (Reuters) - The billionaire owner of basketball's New Jersey Nets took charge of a pro-business political party in his native Russia on Saturday, a move that could help reformists marginalised under Prime Minister Vladimir Putin.
Mikhail Prokhorov, who earned his fortune in Russia's metals industry before buying his way into the U.S. NBA basketball league, said he planned to make the Right Cause party into the main alternative to Putin's long-dominant United Russia.
Right Cause was formed just two years ago and has no seats in parliament after reformist groups were eliminated from the political maintream under Putin, president from 2000-2008.
Prokhorov said in an acceptance speech after being elected Right Cause's leader that he hoped to make it the second-largest party in parliament. Putin's United Russia has 315 seats in the 450-seat State Duma. The Communists are second with just 57.
Right Cause does not describe itself as an opposition party and backs Putin's hand-picked Kremlin successor, President Dmitry Medvedev, for another term.
Most experts believe Putin, who stepped aside after serving two consecutive terms, plans to return to the Kremlin himself in an election next year. Many reformists would prefer Medvedev to stay on, believing he is more sympathetic to their cause.
Prokhorov earned a fortune selling a stake in mining firm Norilsk Nickel before the 2008 financial crisis. He has big stakes in the world's largest aluminium producer RUSAL and Russia's top gold miner, Polyus Gold .
His new role is the highest-profile move into politics by one of Russia's powerful billionaires in nearly a decade.
Tycoons frequently held top political jobs under Boris Yeltsin in the 1990s but have steered clear of politics under Putin, especially since oil magnate Mikhail Khodorkovsky was imprisoned and stripped of his fortune in 2003.
"Our country is called the Russian Federation, but by structure it is an empire. Only presidential power works here, and this kind of governance cannot provide stability let alone development," Prokhorov said in his acceptance speech.
"We need to take back parliament. In the near future become the second largest party, and then later, the first," he said.
The 6-foot-8 (2-metre) Prokhorov told reporters Russia should roll back some of the centralising changes to its political system made under Putin, who abolished independent seats in parliament and elections for regional and big city bosses. Khodorkovsky should be freed on parole, he said.
Analysts said there was a place for a reformist party to return to Russia's political stage, but were doubtful Right Cause could muster enough popularity to place second in a December parliamentary vote.
"In Russia there is demand for a liberal party that would offer more civil liberties, stand against absolute power of the government and corruption, and expend opportunities for business," said political analyst Aleksey Makarkin.
"The question is how to successfully create a party like that. Prokhorov was not involved in politics before. Will he manage to make changes inside the party?" he said. (Writing by Jessica Bachman; Editing by Peter Graff)