Washington – Support among Minnesotans for a proposed copper and nickel mine in the state's northeast corner is slumping, according to a Star Tribune Minnesota poll.

Forty percent of Minnesotans approve of PolyMet Mining Corp.'s proposal, a six-percentage-point decline from a poll conducted in February.

Despite the slide, a statewide groundswell of opposition has not emerged. The percentage of Minnesotans who think the project should be rejected crept up to 23 percent from 21 percent.

Support for the proposal remains strong in northern Minnesota, the region that stands to benefit most from the project near Hoyt Lakes, just east of the Iron Range. The latest results show that 52 percent of residents want the mine approved; only 10 percent say it should be rejected.

But support for the project has taken a hit, even in mining country. The recent poll results represent a 17-point decline in approval from February, when 69 percent of northern Minnesota residents said the state should approve PolyMet's application.

The Minnesota Poll, conducted by Mason-Dixon Polling and Research Inc., interviewed 800 likely voters from Sept. 8-10 by land-line phones and cellphones, with a margin of error of plus or minus 3.5 percentage points.

With PolyMet's promise to bring jobs to northern Minnesota, the mining plan has raised hopes of an economic revival, but its predicted environmental impact has faced criticism from watchdog groups and skepticism from some residents.

"I think that if sulfide mining is as hard to control as it appears to be by looking at the rest of the world, we have to move very cautiously," said Tom Jones, 60, a construction company owner from Ogilvie in central Minnesota who identifies as a Democrat.

More than one-third of Minnesotans still remain unsure — perhaps an indication that people aren't monitoring the public debate on the issue.

PolyMet spokesman Bruce Richardson said the poll results are consistent with the company's research that shows residents are willing to let environmental regulators manage the decision.

Deep divide

Nearly half of Minnesotans — 48 percent — think protecting the environment outweighs the benefit of providing mining jobs; 32 percent side with jobs while 20 percent are unsure, the poll found.

PolyMet's $650 million open pit mine and processing plant would operate for 20 years but could require decades or even centuries of expensive water treatment to protect the lakes and rivers that ultimately drain into Lake Superior.

PolyMet says it would create as many as 360 long-term mining jobs and hundreds of temporary construction jobs. Environmentalists contend that copper mining carries even greater environmental risks to water than the taconite mining that has fueled the Iron Range economy for decades.

"[The poll] results show that Minnesotans are unsure about PolyMet because of the many unanswered questions about the proposal," said Aaron Klemz, spokesman for the Friends of the Boundary Waters Canoe Area, an environmental advocacy group. "As Minnesotans learn more about what is at stake, their concerns continue to increase."

The PolyMet project would occupy space near the edge of the Boundary Waters Canoe Area Wilderness, a treasured Minnesota resource. Many Minnesotans are torn about the trade-off between economic development and potential environmental risks.

"That's a tough one," said Paul Fritze, a 75-year-old Republican and retired teacher from New Ulm in southern Minnesota. "I guess as I see it right now, it provides jobs and that's what we need so badly right now. I don't think you should leave out the environment entirely."

The U.S. Environmental Protection Agency gave plans for the controversial project a passing grade in March, but federal regulators still want a clearer explanation of how pollution problems would be resolved. The state Department of Natural Resources is conducting its own environmental study, with a report due later this year.

Sixty-five percent of Democrats and more than half of independents think that protecting the environment outweighs the potential for job creation; less than a quarter of Republicans do.

The party's candidate for governor, Jeff Johnson, said he would approve plans for PolyMet before the state review is complete, a move that DFL Gov. Mark Dayton called irresponsible.

Staff writer Abby Simons contributed to this report.

Corey Mitchell is a correspondent in the Star Tribune Washington Bureau • Twitter: @C_C_Mitchell