Monthly Medicare premiums for most beneficiaries will rise next year by $3.50, to $99.90, a much smaller increase than had been expected, the Obama administration said Thursday. Administration officials rejoiced at the modest increase, which could pay political dividends to President Obama as he tries to win the votes of older Americans in his bid for re-election. In May, the administration predicted that most beneficiaries would be paying premiums of $106.60 a month in 2012. Administration officials said the smaller increase showed their prudent management of the program, using tools provided by the new health care law to control costs.