For-profit colleges have spent millions of dollars on an advertising campaign opposing the Education Department’s “gainful employment rule,” which would cut off federal aid to programs whose graduates have high debt burdens and low loan-repayment rates. Now, supporters of the proposed regulation are fighting back with an ad of their own. The 30-second spot, which will run tomorrow and Wednesday on MSNBC and Fox News, accuses for-profit colleges of ripping off students and taxpayers.