ORLANDO, Fla., Mar 24 (Reuters) - U.S. Vice President Dick Cheney on Friday rejected charges by Democrats that the Bush administration was mishandling Iraq and said: "If they are competent to fight this war, then I ought to be singing on American Idol." During a campaign stop in Orlando, Cheney predicted that national security would dominate congressional elections in November and sought to rally Republicans amid dwindling popular support for President George W. Bush. In Washington, Democratic National Committee spokesman Luis Miranda called the criticism of his party an attempt to divert attention from the situation in Iraq, which is...