Australians were choosing Saturday between the known and the new at elections that pit a four-term prime minister selling his economic management record against a new-age challenger who rates global warming as his No. 1 priority. Labor Party opposition leader Kevin Rudd held a clear lead in opinion polls over Prime Minister John Howard heading into the ballot, but the margin appeared to be tightening late in the campaign. Polling booths opened at 8 a.m. Saturday (2100 Friday GMT) in eastern Australia and would close 10 hours later, with some 13.5 million people required to vote out of a population of 21 million. Time zones meant western states would continue voting for another three hours.