Sallie Mae lobbied Congress and put forward its own proposal for overhauling the federal student-loan system. After the House vote didn't go their way, the student-loan giant began drumming up grass-roots opposition to the legislation in the towns where its largest facilities are located. If bank-based lending ends, Sallie Mae employees told their neighbors and friends, the company would face a major downsizing. The question is whether the argument, and the 186,092 signatures gathered nationwide in the just-ended petition campaign, will be enough to change the outcome when the Senate takes up the measure,