*CBO says Senate Democrat plan would cost $1 trillion

*Analysis says millions would still need medical cover

*CBO stresses estimates very preliminary (Adds background, quotes)

By Richard Cowan

WASHINGTON, June 15 (Reuters) - A U.S. Senate panel proposal to expand healthcare coverage would increase the federal deficit by about $1 trillion over 10 years and still leave millions without insurance, a congressional analysis said on Monday.

The Congressional Budget Office (CBO) said its calculations were preliminary and stressed that the Democratic-sponsored legislation was still being drafted.

But its estimates could fuel opposition to President Barack Obama's drive for healthcare reform, which critics fear would result in an expensive government takeover of the U.S. healthcare system.

There are now an estimated 46 million people in the United States without any health insurance, and devising a way to get them coverage has become a major goal of Obama's presidency.

In a letter to Senator Edward Kennedy, chairman of the Senate's Health, Education, Labor and Pensions Committee and author of the plan, the non-partisan CBO estimated that once the reform plan was fully implemented about 39 million people would get coverage through new insurance exchanges -- a kind of clearinghouse for medical plans.