House Republicans say their budget proposal would make Medicare work just like the health insurance that covers federal employees, including members of Congress. But a close examination shows the two plans are very different, and the differences help explain why the Republican plan has set off a political uproar. Under the federal employees' health plan, which covers eight million people, the government pays a fixed share of premiums. So the federal contribution generally keeps pace with rising premiums, which in turn reflect rising health costs. No such guarantee exists under the Republicans' plan to transform Medicare, approved by the House on April 15 as part of a budget blueprint to cut federal spending and deficits.