In Rep. Rick Boucher's district in the coal fields of Southwest Virginia, federal schemes to limit pollution are usually about as popular as unemployment. In Rep. Tom McMillen's Maryland district beside the Chesapeake Bay, environmentalism is almost always good politics.

So, when the two considered landmark clean air legislation that passed the House of Representatives this week, it was logical to assume that McMillen would support the proposal and that Boucher would work to kill it.

Logical, but wrong.

The forces of politics combined to put those Democrats in unaccustomed positions on an environmental issue: Boucher was an enthusiastic booster of the clean air bill, while McMillen drew fire from environmentalists for trying to weaken it.

How the pair wound up in those positions demonstrates the high-stakes maneuvers and trade-offs that shape legislation as wide-ranging as the clean air package. Both McMillen and Boucher serve on the House Energy and Commerce Committee, the influential panel that drew up the clean air bill, and both were forced to choose repeatedly among influential but conflicting constituencies.

McMillen describes the clean air debate as "a little like being put in a frying pan. I'm very, very conscious that I have a district that is environmentally sensitive . . . . But I have to make independent judgments."

Boucher has traditionally resisted tightening laws relating to "acid rain," caused in part by smoke from coal-fired electricity generating plants. Any law that discourages the use of coal could cost miners in Boucher's district their jobs.

But Southwest Virginia's mines produce coal that is low in sulphur content and therefore pollutes less than coal from many other parts of the country. Boucher pushed for a provision that encourages the use of low-sulphur coal, an approach incorporated in the bill. He says the plan could create hundreds of jobs in his district.

For Boucher, this represents the best of two worlds. "I'm happy with the legislation," he said recently with a grin. "We have an opportunity to address environmental problems and my district's economic concerns at the same time."

McMillen, on the other hand, usually gets high marks from environmental groups. But they charge that he cast several votes to weaken car pollution standards as part of a deal to obtain his seat on the committee.

McMillen spent several years lobbying for one of the sought-after spots on Energy and Commerce before he was tapped in February. The key to his appointment was committee Chairman John Dingell (D-Mich.), who represents a Detroit area district dominated by car manufacturing and who has historically opposed tougher laws governing tailpipe emissions.

McMillen voted with Dingell and against environmentalists on several car industry issues. "Tom McMillen really wanted to get on Energy and Commerce, and this is the price he had to pay," said Joan Willey of the Sierra Club's Annapolis office. "John Dingell wields enormous power. And Representative McMillen has sided with Dingell on clean air."

McMillen said there was no quid pro quo for his votes and pointed out that he supported the clean air bill on the House floor. "I'm proud of my environmental record," he said.

Although McMillen and Boucher are in different positions on the issue, their political careers are decidedly similar. Both are philosophical moderates. Both are young -- Boucher is 43 and McMillen turns 38 today -- and are junior House members who aspire to leadership roles. Both pursued seats on the Energy and Commerce Committee in part to enhance their prestige.

But their votes on clean air are influenced by their home districts, which contrast sharply.

Boucher's district begins in the hills beyond Roanoke and sprawls outward to Appalachia and the Cumberland Gap. It is rural, poor and heavily dependent on mining.

A coal industry consulting firm has estimated that the clean air bill could create more than 2,000 badly needed jobs in the district; no version of the bill would take away jobs.

Business representatives and political activists in both parties say that Boucher's performance on the clean air bill has turned a potential political disaster into an asset. Bonnie Elosser, a Southwest Virginia Republican activist, says that when the legislation was being drafted several months ago, "everybody in our party was just licking their chops, saying this will be great for us.

"But quite frankly, what you've been hearing lately if you talk to people down at Hardee's or somewhere is that this will help our area," Elosser said. "I don't look for this issue to give {Boucher} a problem."

McMillen's district includes all of Anne Arundel County and parts of Howard and Prince George's counties. It is increasingly urban, sophisticated and wealthy. And the proximity of the Chesapeake Bay -- Anne Arundel has more than 400 miles of shoreline -- makes the environment a paramount political concern.

McMillen was first elected in 1986 by a margin of less than half a percentage point. One vocal source of support was environmental groups.

McMillen's stand on two issues drew their ire. He helped kill a proposal to require cleaner-burning gasoline in about 30 severely polluted urban centers, including the Washington and Baltimore areas. He also voted to ease the minimum warranty requirements for car pollution equipment, a proposal that was rejected on the House floor.

"I think he really wants to do the right thing. But we're going to hold him accountable for his votes," said Mary Rosso, of the Maryland Waste Coalition.

McMillen wasted no time taking a stand that brought him closer to environmentalists. A compromise proposal to require cleaner-burning gasoline in the nine most polluted areas, including Baltimore, was passed on the House floor and McMillen publicly supported it.

Political activists say the clean air debate probably will have limited repercussions for McMillen; like Boucher, he is a heavy favorite to be reelected in November.