Campaigning for Russia's parliamentary election ended on Friday with President Vladimir Putin's party on course for a landslide victory and opposition groups voicing fears of widespread ballot-rigging.Putin is leading the United Russia party into Sunday's vote and will use the big majority it is expected to win to cement his grip on power even after he steps down from the presidency at the end of his second term next year. Most Russian voters credit Putin with restoring stability and economic growth after the chaos of the 1990s. Opinion polls indicate his party will pick up about 60 percent of the vote on Sunday, with its nearest rivals trailing far behind.