A steady stream of voters, some from storm-scarred neighborhoods and others by the busload from evacuee havens across the nation, cast their first ballots since Hurricane Katrina in a crucial election Saturday to decide who oversees how much of their city is rebuilt. Incumbent Mayor Ray Nagin predicted he would lead the field, but he faces 21 challengers, including the state's lieutenant governor. If none gets more than 50 percent, a runoff between the top two vote-getters will be held May 20. Of the city's 297,000 registered voters, about 20,000 cast ballots early by mail or fax or at satellite voting stations set up around the state. Turnout figures would not be available for hours but a spokeswoman for Secretary of State Al Ater, the state's top election official, described turnout at randomly surveyed precincts as "steady."