A week before a Senate vote, the Obama campaign on Monday ramped up its advocacy of the “Buffett Rule,” which would impose a minimum effective tax rate of 30 percent on the wealthiest Americans. Campaign manager Jim Messina held a conference call with reporters that included endorsements by Democratic lawmakers, and announced a new website that offers practical examples of how Buffett Rule tax revenue could be spent. Republicans argue the rule’s deficit-reducing power is overstated.