Thousands of protesters marched around the White House Sunday in opposition to the Keystone XL pipeline, which environmental groups say would worsen the risks of climate change by encouraging development of Alberta’s oil sands.

Organizers said 35,000 people from across the country attended the rally in Washington, held in biting winds and near-freezing temperatures. That would make it the largest climate-change protest in U.S. history, according to sponsors, who said it signals the rise of a national movement demanding action on global warming as President Barack Obama weighs whether to approve the $5.3 billion pipeline.

“Twenty-five years from now, nobody is going to look back at our era and say, ‘Boy, I wonder how that fiscal cliff thing came out,’” Bill McKibben, founder of 350.org, an environmental group dedicated to fighting climate change and one of the sponsors of the rally, told reporters before the event. “Everyone is going to look back and say, ‘Well, the Arctic melted, and then what did you do?’”

The rally should show Obama that he has to “the support he needs to block this pipeline,” McKibben said.

The project would cross six states and link Alberta’s oil sands to refineries along the Gulf Coast. The U.S. State Department is reviewing the pipeline because it crosses an international border. Secretary of State John Kerry has said that the review would be completed soon.

In January 2012 the Obama administration rejected a proposed route for the pipeline after concerns were raised about the impact of the project on an ecologically sensitive area in Nebraska. The route now under consideration was submitted in September 2012.

Obama was playing golf in Florida during Sunday’s protest.

Environmental Symbol

Environmentalists oppose the pipeline because it would encourage development of Alberta’s oil sands, the mining of which releases more carbon dioxide, which most scientists believe is a greenhouse gas, than conventional drilling.

TransCanada Corp.’s project has become a symbol of a larger fight over environmental protection and economic development in the same way drilling in Alaska’s Arctic National Wildlife Refuge was a decade ago.

“It will be a punch in the gut to all the activists who worked so hard to get the president elected” if Keystone is approved, said Michael Brune, the executive director of the Sierra Club, a San Francisco-based environmental group that also sponsored Sunday’s protest.

Supporters in the oil industry said the project will create thousands of jobs and improve U.S. energy security by lessening the need to import oil from less friendly countries.

Protesters filled an area on the National Mall near the Washington Monument, carrying signs urging Obama to reject the pipeline. “Tar Sands = Game Over” read one.

Wind Chill

American flags and flags with pictures of the Earth on them whipped in the wind, which the National Weather Service said reached more than 30 miles per hour at times, dropping the wind chill to 21 degrees Fahrenheit (-6 degrees Celsius).

After a series of speeches from organizers, protesters marched in a column around the White House, where they stopped to chant “We are unstoppable; another world is possible.” They stood along 15th Street, east of the White House, holding placards that were black on one side to symbolize the pipeline.

Claire Silvers, 62, said she came by bus from Cambridge, Massachusetts, hoping to demonstrate widespread support for action on climate change.

Sarah Fuelleman of Madison, Wisconsin, took a bus to Chicago and then a train for 17-1/2 hours to Washington.

“It’s been raining in Wisconsin in January,” she said. “I can’t imagine having people suffer from the realities of climate change.”

Second Rally

Sunday’s protest was the second large rally in opposition to the pipeline. In November 2011, critics encircled the White House in an event that drew at least 12,000 people, organizers said.

Sunday’s rally drew nearly three times that number, according to sponsors, and shows that concern about warming temperatures is rising after the hottest year on record and extreme weather events such as Superstorm Sandy.

Environmentalists have been encouraged by Obama’s pledges in recent weeks to tackle global-warming risks -- first in his inaugural address and then again last week in the State of the Union speech, telling Congress: “We must do more to combat climate change.”

Last week Senators Barbara Boxer, a California Democrat, and Bernie Sanders, a Vermont independent, introduced legislation that would tax carbon emissions.

Republican Opposition

In 2010, Senate Republicans blocked action of climate- change legislation that the House, then controlled by Democrats, had passed. Many Republicans oppose federal efforts to limit carbon emissions, contending that doing so would hurt the economy.

“I think the polluting industries pretty much have Congress locked up in terms of being able to do anything on our own,” Senator Sheldon Whitehouse, a Rhode Island Democrat who attended Sunday’s rally, said in an interview. “That’s one of the reasons today is important -- because it reflects the voices of people across the nation that they’re fed up with the barricade in Washington and that this is an issue that can’t be overlooked.”