It sounded simple enough on the campaign trail: Free the government to negotiate lower drug prices and use the savings to plug a big gap in Medicare's new prescription-drug benefit. But as Democrats prepare to take control of Congress, they are struggling to keep that promise without wrecking a program that has proven cheaper and more popular than anyone imagined. House Democrats have vowed to act quickly after taking power in January to lift a ban on Medicare negotiations with drugmakers, which they hope will save as much as $190 billion over a decade. But House leaders have yet to...