WASHINGTON (AP) _ When it comes to the environment, Sen. Max Baucus is a man in search of middle ground.

Most of the time, the Montana Democrat - who is expected to chair the Environment and Public Works Committee starting in January - manages to balance the interests of environmentalists and business.

Baucus is ''essentially an environmentalist,'' says Caleb Marshall, the senator's press secretary. ''But he's also from a state where the economy depends on extraction industries - mining and timber.''

That conflict, as much as any, has caused him to gain only mediocre approval ratings from environmental groups over the years. The League of Conservation Voters gave him a 69 percent approval in the last Congress and a meager 32 percent in 1989-90.

Nevertheless ''he's generally regarded as a friend of environmental causes,'' says Michael McCloskey, chairman of the Sierra Club.

The chairmanship of the Environment and Public Works Committee opened up with the death of Sen. Quentin Burdick, D-N.D., earlier this year. But Sen. Daniel Patrick Moynihan, D-N.Y., next in seniority, is expected instead to become chairman of the Finance Committee.

That leaves Baucus in line to head the environmental panel. His influence also will be increased by his elevation to the No. 2 spot on the Finance Committee, whose chairman, Texas Sen. Lloyd Bentsen, was selected last week by President-elect Clinton as treasury secretary.

A quiet-spoken Democrat from a largely Republican family of Montana ranchers, Baucus, 51, has put much of his legislative energies into environmental issues in recent years.

He was key in guiding the complex and contentious clean air bill through the Senate in 1990 and this year gained the wrath of industry by trying to impose minimum recycling requirements.

He has argued for stronger measures to deal with global warming, believes U.S. trade policies should be used to foster environmental improvements in other countries such as Mexico, and was among a small group of senators who were key in blocking attempts to open the Arctic National Wildlife Refuge in Alaska to oil development.

Those efforts have won him praise from environmentalists.

When issues hit closer to home, however, Baucus has been less strident as he walks a careful line between protecting natural resources and developing them.

He gained the wrath of some environmentalists, for example, by opposing sweeping reforms of an 1872 mining law, though he later agreed some changes in the law are needed.

He also was opposed by many environmentalists in his attempt to forge a compromise on a decade-long dispute over whether to declare millions of pristine Montana acres wilderness. While his compromise would have protected 1.2 million acres of Montana, it also would have opened nearly 4 million acres to timber and mining interests. The compromise didn't make it through Congress.

Scott Shotwell, a lobbyist for the timber industry, says Baucus is ''a person who comes from the West and understands land use issues as well as environmental issues.''

Baucus grew up on his family's 125,000-acre spread near Helena, Mont., and later attended Stanford University where expressed little interest in politics. ''I thought it was dirty, corrupted and tainted,'' he once told an interviewer.

Yet, he soon went to Washington where he worked as a government lawyer before returning to Montana. He was elected to Congress as a House member in 1974 after a campaign that included a 631-mile walk around the state.

Baucus, a jogger and occasional marathon runner, was elected to the Senate in 1978.

Considered by many of his colleagues as reserved and mild mannered, Baucus surprised some guests to his 50th birthday party a year ago by pulling on a leather jacket and firing up a motorcycle in the ballroom of a Washington hotel. He and his wife, Wanda, rode the cycle around the room as the band played ''Leader of the Pack.''

In the upcoming 103rd Congress, the Leader is moving up.