CORALVILLE, Iowa — Presidential candidate Bernie Sanders let loose a volley of sharp attacks against Michael Bloomberg on Saturday night, accusing the former New York mayor in his bluntest language yet of positioning himself to buy the election and vowing to stop him from doing so. “Tonight we say to Michael Bloomberg and other billionaires: Sorry, you ain’t gonna buy this election,” said Sanders, I-Vermont, prompting loud cheers from supporters. The fiery comments, which marked an escalation from earlier and less direct criticism from Sanders, underscored how Bloomberg’s recent steps toward entering the Democratic primary have roiled the contest.