(Reuters) - A divided Arizona Senate passed a key piece of President Barack Obama's Medicaid expansion agenda on Thursday, handing Republican Governor Jan Brewer a policy victory over fierce opposition from conservatives in her party. By an 18-11 vote, the Senate approved the bill with the backing of a bipartisan coalition of lawmakers that will add hundreds of thousands of the state's poorest residents to the Medicaid healthcare rolls, but was opposed by conservative Republicans. The state House of Representatives approved the same measure by a 33-27 vote early on Thursday after a marathon debate.