House Democratic leaders are scrambling to cope with the fallout if, as seems increasingly likely, Ways & Means Committee Chairman Dan Rostenkowski (D-Ill.) is indicted in the House post office scandal. Under House rules, ranking Democrat Sam Gibbons (D-Fla.) would become interim chairman pending Rosty's trial, and the leadership is worried that the quirky, 73-year-old Gibbons isn't up to the crucial job. The biggest fear is that Gibbons, an uncompromising free-trader, lacks the finesse to get the North American Free Trade Agreement through the House. One option under discussion is setting up a sort of collective leadership for the crucial panel. Gibbons would wield the gavel, but senior members including J.J. Pickle (D-Tex.) and Charles B. Rangel (D-N.Y.) would be the powers behind the throne. According to one Ways & Means source, some Democratic leaders were spooked recently at a closed-door session when Gibbons sidestepped detailed questions about the politically sensitive budget bill and instead plugged the merits of one of his pet notions, a value-added tax. Liberals, meanwhile, fret that Gibbons is too close to business lobbyists and too unpredictable to be a reliable member of the House leadership team.