Senator Bill Nelson of Florida wants to expand health insurance coverage because one in five Floridians is uninsured. And as a member of the Senate Finance Committee, he can help shape legislation. But the bill taken up this week by the committee would cut Medicare payments to insurance companies that care for more than 10 million older Americans, including nearly one million in Florida. Nelson said he had received 56,000 telephone calls, letters and e-mail messages on the legislation since June.