Organized labor leaders have been among Democrat Elizabeth Warren’s earliest and most ardent supporters, funneling thousands of dollars into her campaign, offering her a high profile speaking platform in the run-up to her election announcement last fall, and introducing her to voters in South Boston this week. But harnessing that energy - to woo back the legions of rank-and-file union members who supported Republican Scott Brown in 2010 - also carries risk, as some voters view unions as special interests, intent on preserving perks at the expense of businesses and taxpayers.