Democrat Barack Obama on Monday ridiculed the idea of being Hillary Rodham Clinton's running mate in the US presidential elections, saying voters must choose between the two for the Democratic nomination. The Illinois senator used his first public appearance of the week to knock down the notion that he might accept the party's vice presidential spot on the fall ticket. He noted that he has won more states, votes and delegates than Clinton so far. "I don't know how somebody who is in second place is offering the vice presidency to someone who is first place," Obama said, drawing cheers and a standing ovation from about 1,700 people in Columbus, Mississippi. Saying he wanted to be "absolutely clear," he added: "I don't want anybody here thinking that 'Somehow maybe I can get both"' by nominating Clinton as president and assuming he would be her running mate. "You have to make a choice in this election," he said.