The Arizona legislature passed a bill that protects anonymous political spending Thursday, less than a month after Tempe, Arizona, residents voted overwhelmingly to increase transparency on that type of spending in local elections. The battle between city and state opens a new front in the national debate over so-called dark money" in politics; it's also the first time a state has moved to ban local governments from shining light on secret spending. This type of spending is most consequential at the local level, experts say, because since local elections are relatively cheap and receive little media coverage, ads or mailers backed by just a few thousand dollars of dark money can easily dominate a campaign. More than 90 percent of Tempe voters cast ballots for an amendment to the city's charter requiring all groups spending more than $1,000 on local races to disclose their donors in March. In response, the Republican-controlled Arizona Legislature passed a bill last week that bans local governments from requiring non-profit groups, the most common dark money vehicles, to disclose the source of their funding. Thanks to the U.S. Supreme Court's 2010 Citizens United decision, these groups can spend an unlimited amount of money on elections, so long as they don't coordinate with candidates and their parties. Anonymous spending has surged. As a result, cities and states have passed their own political disclosure laws. Denver and Philadelphia, for example, have passed laws requiring disclosure of dark money.