WASHINGTON – President Bush called members of Congress, including Sen. Susan Collins, to the White House on Wednesday to discuss different versions of a Medicare prescription drug bill, as lawmakers began considering the new drug benefit on the Senate floor.

Both Collins and Sen. Olympia Snowe, Maine’s other Republican senator, are expected to play key roles in the debate over one of Congress’ perennially thorny topics.

House Republicans advanced their Medicare plan in the House Ways and Means Committee on Tuesday, a version that would rely heavily on the private sector to deliver the new prescription drug benefit. Hours of partisan debate on the panel suggested the bill will face contentious divisions when it arrives on the House floor.

The president has said he wants a completed bill on his desk by July 4, a deadline that Snowe said would require an “ambitious” conference to work out the differences between the versions expected to be passed by the House and Senate.

Collins said she was confident that an agreement on a drug benefit would be reached this year, despite the partisan divide that has impeded the bill’s passage in the past. “Last year, you had some people on both sides trying to score political points rather than work cooperatively,” Collins said. “Both the administration and Democrats and Republicans understand that we must take advantage of this unprecedented opportunity.”

The new program would be paid for by $400 billion set aside in the budget, as well as a $35 monthly premium paid by seniors who participate, Collins said. The premium will be means-tested, meaning that seniors whose income level is below 165 percent of the poverty level would not have to pay the fee, she said.

Collins said that seniors need to realize that any prescription drug benefit program is not likely to be in place until 2005. She added that the final legislation is likely to provide seniors with a discount drug card that immediately will save them 15 percent on their drug bills, and that low-income seniors would receive $600 per year to help offset drug costs.