WASHINGTON -- As the Senate prepares for a showdown over energy policy, Senator John F. Kerry of Massachusetts is leading the Democratic fight on the two most hotly contested elements of the package - whether to drill along the Alaskan coastline and whether to force US auto makers to produce vehicles that are more fuel-efficient.

Kerry, a longtime environmentalist, has positioned himself squarely opposite the Bush administration and some labor unions, a traditional Democratic constituency.

The debate, expected to begin on the Senate floor this week, provides a national platform for Kerry, who is mulling a 2004 presidential run. But while some analysts believe Kerry could gain valuable exposure as a leading environmentalist in his party, others wonder whether the Massachusetts lawmaker will alienate key parts of the electorate, such as auto workers and owners of low-mileage sport utility vehicles. "It seems to me his appeal is far less than, say, Mike Dukakis, nationwide. And I don't see this helping that," said Myron Ebell, a climate-change specialist at the conservative Competitive Enterprise Institute.

But Mark Mellman, a Democratic consultant, said Kerry was tapping into two potentially important themes for the 2004 election: a desire for cleaner air and water, and a sense that the Bush administration may be too close to the energy industry.

"There's a mythology about this, that everybody in the affected world, who drives an SUV or works in the auto industry, doesn't favor tighter restrictions on fuel emissions," said Mellman, adding that his polling shows more than 80 percent of Americans want cars to be more fuel-efficient. Energy and the environment, Mellman said, are President Bush's Achilles' heel.

The Democrats' bill, brought to the Senate floor by majority leader Thomas A. Daschle of South Dakota, places greater emphasis on renewable energy and conservation than Bush's energy plan. In addition to raising the fuel efficiency standard to an average of 35 miles per gallon by 2013, the bill includes $16 billion in tax credits, evenly split between fossil fuels and nuclear power, and efficient and renewable sources of energy.

The bill also includes $10 billion in loan guarantees to encourage the construction of a natural-gas pipeline from oil wells in Alaska to the lower 48 states. Democrats argue that such a pipeline would not only bring trillions of cubic feet of clean-burning natural gas into the mainland, but would also create more jobs than drilling in the protected portion of the northern Alaskan slope.

The legislation also would boost the Low Income Home Energy Assistance Program from $2.25 billion to $3.4 billion annually.

In August, the House passed an energy plan that more closely resembled Bush's, with three dozen Democrats joining most of the Republicans voting in favor. The House bill included a far smaller fuel-efficiency increase. The focus of the bill was on tax cuts, with $33.5 billion over 10 years aimed at spurring oil, gas, and "clean coal" production, as well as some conservation. That measure also increased funding for low-income energy assistance.

The most prominent difference, however, is that the House bill would open a portion of the Arctic National Wildlife Refuge for oil exploration. The Senate bill has no such provision, and Kerry has vowed to filibuster any attempt to insert it into the bill.

Kerry says he is optimistic about his chances of success, but it appears less likely he will prevail in his effort to raise the fuel-efficiency standard known as the Corporate Average Fuel Efficiency, or CAFE. "We know what an uphill climb this is," Daschle said. "We're not fooling anybody."

John Podesta, a former chief of staff to President Clinton who is a board member of the League of Conservation Voters, added: "When I was in the White House, we always had a hard time counting to 34 [votes] on CAFE. If he can get to 50, that will be some achievement."

His push to raise the efficiency standards sets Kerry in opposition not only to the White House, which maintains that the National Highway Traffic Safety Administration should handle the issue instead of Congress, but also to the United Auto Workers, a powerful union in early presidential primary states.

Kerry has been talking with Senator John McCain, Republican of Arizona and author of a competing bill. But asked whether he would trade a ban on drilling in the Arctic refuge for dropping the fight on fuel standards, Kerry said, "You can't take something that has a sensible policy goal and legitimacy in its rationale and trade it for something that has no positives and no legitimacy."

Opponents of the Senate Democrats' bill have targeted Kerry. The Teamsters union ran a newspaper ad in Tampa blasting Kerry's opposition to drilling. The Sierra Club has vowed to respond soon in defense of the senator.

"With all due respect to the senator, he probably ought to chat with Al Gore and find out why the ticket in the presidential election managed to lose the five electoral votes in West Virginia," a margin that could have given Gore the White House, said Jerry Hood, the Teamsters' lead lobbyist on the issue.

"He'll find that regardless of organized labor's endorsement of Al Gore and that ticket, our rank-and-file members, the blue-collar workers of West Virginia, those that work in the trenches, were fearful of losing their jobs because of the environmental issues of that campaign."

But other Democrats say that Gore, whose book "Earth in the Balance" marked him as an environmentalist, erred by not making the issue a central part of his campaign. Instead, Gore hammered away on Social Security and health care.

"An awful lot of people thought Gore made a mistake in putting it aside," said Jim Jordan, executive director of the Democratic Senatorial Campaign Committee and a former Kerry aide.

For Kerry, the debate "sets him up as not only a target for the opposition, but it sets him up as being a party leader - maybe the party leader - on issues the Democrats enjoy the most strength on," said Scott Stoermer, spokesman for the League of Conservation Voters. "You don't see the Teamsters targeting any other Senate leaders . . . That's just an indicator of where Kerry fits in in the Democratic hierarchy."

Charles Manning, a Boston-based GOP consultant, noted that Kerry made a name for himself by raising the acid-rain issue, and said he might benefit again from rallying environmentalists.

"If you can get these people up front, that's a terrific move for him," Manning said.