In 2008, Courtney Hight fell in love with Barack Obama’s message of hope and change, especially his stalwart support of renewable and alternative energy. She worked long hours as the youth vote director for his campaign in Florida.

But lately the young activist has started to feel that President Obama isn’t quite the man she fell for. During his energy security speech at Georgetown University in March, when he said oil drilling and clean coal would help power America’s energy future, Hight said she accepted what friends told her for weeks: Obama changed.

On Friday, Hight and 10,000 other young clean-energy advocates will open the third Power Shift conference at the Washington Convention Center in the District. The three-day climate summit takes place every other year.

But instead of endorsing the president’s energy policy, as in 2009, they plan to lambaste it, saying that Obama is siding with what they consider to be the dark side — big oil and coal-fired power plants. Organizers are planning a demonstration Monday with 5,000 participants outside the White House.

“When I looked at that energy security speech, it seemed like something BP wrote,” said Hight, 31, of Scottsdale, Ariz., who is co-director of Power Shift 2011. “We want to make sure the president is seeing that we’re done with this. We need them to draw a line in the sand. We need him to stand up to the polluters.”

Considering the political environment in Washington, where congressional Republicans are fighting Obama’s every step, some say Power Shift’s demands are unrealistic.

And Obama’s energy security speech wasn’t devoid of messages that Power Shift’s organizers favor. He said he wanted to cut America’s oil dependence by a third in the next decade, put a million more electric vehicles on the roads by 2015 and help Americans upgrade their homes and businesses with energy-efficient building materials that could save them tens of billions of dollars a year.

But when Obama said his administration has approved 39 new shallow-water drilling permits and an additional seven deepwater permits in recent weeks, following the disastrous Deepwater Horizon oil spill last year in the Gulf of Mexico, it was akin to dragging his fingernails across a blackboard for his base of young environmental voters.

“I worked for Barack Obama for years,” said Hight, who claimed that her organizing in Tampa helped drive hundreds of thousands of voters to the polls. “When I saw that, it almost made my stomach drop. When I watched that speech, that’s when I changed. It flipped me.”

The president continues to enjoy high approval ratings from Americans ages 18 to 29, according to a recent Washington Post/ABC News poll. Several workers at Power Shift’s command center in downtown Washington said they will probably vote for him.

But their vote isn’t really the issue. It’s their desire to take time off from their jobs, knock on doors, drive enthusiasm on Facebook, Twitter and other social networks, and work young people into a get-out-the-vote frenzy for Obama.

“He has my vote, but only by default, by virtue of the clowns on the other side,” said Derrick Evans, 44, of Gulfport, Miss., referring to Republicans who favor oil drilling and oppose Environmental Protection Agency regulations on greenhouse gas emissions from power plants. “If I were [Obama], I wouldn’t want that to be the glue that adheres me.”

As he monitored the Twitter traffic of people who said they were on their way to the conference, Jeff Mann, 25, of Bloomfield Hills, Mich., said the frustration with the president’s energy policy was evident.

“There are hundreds of tweets asking Obama not to go with big polluters,” said Mann, the online director for the Energy Action Coalition, 50 youth-led environmental and social justice organizations that created Power Shift. “More than a few people are going to support Obama, but they’re not excited about it. I haven’t seen anyone on my list who wanted to go out after Power Shift and join the Obama 2012 campaign.”

Jenna Garland of Woodstock, Ga., who’s recruiting youth to the conference, said her support for Obama hinges on whether he continues to embrace oil, coal and natural gas, which is extracted using chemicals that environmentalists say endanger fresh water sources. “That is the thing that will determine whether I will take the time off and do the door knocks,” said Garland, 26. “I may have a real hard time getting motivated to do that for him.”

At Power Shift’s command center on M Street, young volunteers were hard at work preparing for the summit, where former vice president Al Gore and EPA Administrator Lisa P. Jackson are scheduled to speak.

Jennifer Ridder, 24, of Denver, said she worked from 9:30 a.m. Wednesday to 1 a.m. Thursday assigning rooms at the convention and working on lodging.

Ridder said most participants can’t afford to rent a hotel room, so she’s lining up space at hostels and Arlington camp sites. She’s also encouraging homeowners to let them sleep in spare beds and back yards.