As Democrats strained to win over crucial holdouts on the way to narrow, party-line approval of global warming legislation, they were dogged by a critical question: Has the political climate changed since 1993?

Veteran members of both parties vividly remember when many House Democrats, in the early months of the Clinton administration, reluctantly backed a proposed B.T.U. tax — a new levy on each unit of energy consumed — only to see it ignored by the Senate and seized as a campaign issue by Republicans, who took control of the House the next year.

“A lot of Democrat members got burnt on that vote,” warned Representative John A. Boehner of Ohio, the House Republican leader, who called the climate change measure the defining vote of this, the 111th, Congress.