Congress entered the final phase of crafting a clean air bill yesterday when House and Senate conferees began the difficult task of reconciling nearly 1,400 pages of legislation.

The bills vary significantly in their prescriptions for acid rain, urban smog and toxic industrial emissions, setting the stage for a potentially bruising fight.

But in their initial face-off under the ornate, cathedral ceiling of a House caucus room, the 147 conferees focused on a common concern: time.

With fewer than 40 working days before Congress plans to adjourn, Sen. Max Baucus (D-Mont.), head of the Senate delegation, urged his House counterpart to plan for the next meeting as soon as possible, noting that "human nature is to procrastinate."

"I've not found that in the House," Rep. John D. Dingell (D-Mich.), the House leader, replied stonily.

The exchange reflects the contrasting sense of urgency in the two chambers.

Both passed bills last spring with new requirements for cleaner motor fuels and tough controls on autos, utilities and manufacturing plants. While Senate Majority Leader George J. Mitchell (D-Maine) moved swiftly to appoint his conferees, House leaders delayed their selection until earlier this month.

Dingell, chairman of the Energy and Commerce Committee, is a longtime foe of clean air legislation. According to House sources, he sought to stack the House delegation with political allies but failed to get the approval of House Speaker Thomas S. Foley (D-Wash.), resulting in a time-consuming dispute.

The upshot was that Foley appointed an additional two pro-environmental conferees and dozens of part-time participants from committees with cross-jurisdiction.

But the 26 members from Energy and Commerce remain the core of the 138-member House delegation, and they are predominantly controlled by Dingell or Rep. Norman F. Lent (N.Y.), the Republican linchpin of the committee's pro-industry coalition.

"A real danger is that those who want a weak bill will use delay as a threat to get their way," said Richard Ayres, chairman of the Clean Air Coalition.

The Senate delegation consists of Mitchell, Baucus and five members of the Senate Environment and Public Works Committee, which passed the most far-reaching version of clean air legislation last year, only to have it watered down on the floor in March.

Mitchell was forced to compromise with the Bush administration to get a bill passed in the Senate. But he is not bound to the weaker measure in conference, freeing him to seek tougher controls.

Dingell does not have the same flexibility. On several issues, including controls on airborne toxics and auto and industrial sources of smog, he is committed to compromises worked out in committee with Rep. Henry A. Waxman (D-Calif.), leader of the environmental bloc.

One issue over which Dingell and Mitchell are expected to clash is the program of gasoline alternatives for centrally fueled fleets of vehicles, such as taxis and police cars. The Senate version is considered tougher because it calls for reductions of cancer-causing as well as smog-generating emissions and phases in strict cutbacks sooner than the House.

On acid rain, both bills aim to halve and cap pollutants by the year 2000. But the methods of achieving these objectives vary, with the House devising more ways to ease costs for dirty utilities in the Midwest and Appalachia.