IOWA CITY — When he was the top federal prosecutor in southern Iowa, Nicholas Klinefeldt won praise for reducing the use of mandatory minimum sentences and enhancements that he argued put too many nonviolent drug offenders in prison for too long.Now the office that Klinefeldt once led is using those tools against his 74-year-old father, Michael, who faces a mandatory prison sentence of 10 years if convicted on drug charges that were brought this month.Michael Klinefeldt was sentenced in 2003 to a mandatory 10-year term after he was convicted of a low-level meth offense — a punishment that has been credited with helping shape his son’s advocacy for what he calls a smarter, fairer approach.He was midway through his sentence when President Barack Obama in 2009 nominated Nicholas Klinefeldt, a defense lawyer with ties to the Democratic Party, to be U.S.