Democratic Congressional leaders, bowing to unease among lawmakers and governors in their own party, have suggested scaling back a plan to tax top earners to pay for healthcare legislation and signaled a retreat from their ambitious timetable. The House speaker, Nancy Pelosi, suggested revising the tax-raising provisions, one of the most contentious parts of the House bill, which would impose a surtax on high-income households. Pelosi said she would prefer that fewer people had to pay the tax, which was approved by the Ways and Means Committee.