The Senate bill is likely to phase out the Affordable Care Act’s Medicaid expansion more slowly than the House version. It is also expected to include larger tax credits to help older Americans buy health insurance.

The legislation will be considered in the Senate under an expedited procedure that precludes a Democratic filibuster and allows passage by a simple majority. But, Republicans say, Democrats will still be able to offer numerous amendments once the bill is on the Senate floor.

It is not unusual for lawmakers to draft major legislation in private, but they usually refine, debate and amend it in open committee sessions. The House bill to repeal the Affordable Care Act did not receive a hearing, where outside experts could have testified. But lawmakers dissected its contents and were able to propose changes at three stages: in the Ways and Means, Energy and Commerce, and Budget Committees.

Senate Republican leaders evidently think their back-room approach gives them the best chance to devise a health care bill that can squeak through the Senate, given their narrow majority and the policy differences in their conference.

However Republicans feel about their coming bill — and they are far more comfortable criticizing the Affordable Care Act than talking up the virtues of their still-incomplete replacement — the process playing out in the Senate is quite different from the way Democrats went about passing the Affordable Care Act.

The Senate health committee approved its version in July 2009 after considering hundreds of amendments over 13 days. The Senate Finance Committee cleared its version in October 2009, after more than a year of hearings, round-table discussions and other spadework. A group of Democrats and Republicans from the Finance Committee had met for months behind closed doors, trying — but ultimately failing — to draft bipartisan legislation.