WASHINGTON, Sept. 11 - Medicare has suddenly emerged as a volatile issue in this year's elections, as Democrats are vowing to roll back a sharp increase in premiums announced this month and the Bush campaign is seeking to blame lawmakers, including Senator John Kerry, for the rise. The trading of accusations reflects efforts by both parties to seek advantage with older voters. Democrats, hoping to reclaim an issue central to their success in past elections, said they would try to block the 17.4 percent increase that will come out of Social Security checks next year.