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The decline in oil prices may be depriving Russian President Vladimir Putin of his biggest ally. Oil has been the key to Putin's grip on power since he took over from Boris Yeltsin in 2000, fueling a booming economy that grew 7 percent on average from 2000 to 2008. Now, with economic growth slipping close to zero, Russia is reeling from sanctions by the U.S. Putin, whose popularity has been more …