LAS VEGAS - The leading Democratic presidential candidates are united on the government's Yucca Mountain nuclear waste storage plan: They'd scrap it.Their vigorous opposition to the project reflects Nevada's importance as one of a handful of states that will lead off voting in January for the Democratic and Republican nominations. Few local issues are as unpopular with Nevadans as the waste dump.The Democrats have just one problem - their records keep getting in the way. Front-runner Hillary Rodham Clinton has created suspicion in some corners of the anti-Yucca lobby because she's refused to rule out expansion of nuclear power as …