In July, Clinton expressed "doubts" over Arctic drilling, saying, "I don't think it is a necessary part of our overall clean-energy climate-change agenda," in an interview with NH1 News.
But the 2016 contender's silence on hot-button environmental issues, such as whether the controversial Keystone XL oil-sands pipeline should be approved, has frustrated environmentalists who fear that she will not do enough to rein in the greenhouse gases that scientists say are driving global warming.
Questions over where exactly Clinton stands on Arctic drilling have also proved to be a sore subject in the campaign's relationship with the climate movement.
Clinton has also long opposed onshore drilling for oil in the Arctic National Wildlife Refuge.
The candidate's democratic 2016 challengers on the Left, Bernie Sanders and Martin O'Malley, both oppose Arctic drilling.
But Clinton's comment nevertheless marks a key break with Obama, whose Interior Department this week gave long-sought permission to Shell to drill into oil-bearing zones in the Chukchi Sea off Alaska's northern coasts. The company had already begun preliminary drilling work.
Federal regulators estimate that the Chukchi Sea could hold more than 15 billion barrels of recoverable oil and large natural-gas resources, too. And the adjoining Beaufort Sea is believed to have huge subsea hydrocarbon pools, too.
But green groups strongly oppose drilling in the Arctic region, which is home to polar bears, bowhead whales, and other endangered or fragile species, arguing that it's too difficult to contain potential spills despite Shell's claim of robust safety and response measures.
Beyond long-standing fears over the potential for a catastrophic spill, the Arctic drilling fight has more recently become part of a much broader battle over climate change.
In May, environmental activist and founder of the grassroots green group 350.org Bill McKibben slammed Obama's decision to let Shell drill for Arctic oil in a New York Times op-ed, calling it another form of "climate denial."
"The Obama administration's decision to give Shell Oil the go-ahead to drill in the Arctic shows why we may never win the fight against climate change. Even in this most extreme circumstance, no one seems able to stand up to the power of the fossil-fuel industry. No one ever says no," McKibben wrote.
Companies including ConocoPhillips and Statoil hold Arctic offshore leases that they have not yet sought to develop. While Shell is the only company currently seeking to drill offshore in the U.S. Arctic, Clinton's stance will likely have broader effects on the industry if elected.
"When she said that given what we know now, offshore drilling in the Arctic is not worth the risk, she was referring to both new and existing leases," a campaign aide said.
Obama's Arctic offshore policies have sought to blend new protections with limited development.
The Interior Department is crafting new oil-spill prevention and response rules for companies operating in Arctic waters off Alaska's coast.
New Arctic lease sales are tentatively scheduled in 2016 and 2017, and a draft Interior Department plan released early this year also envisions auctions of drilling rights in Arctic waters in 2020 and 2022. However, Obama has also placed major areas of the Arctic seas off-limits.
This story is breaking and has been updated.