Former Mich. Gov. Jennifer Granholm would also bring political heft to the department, although her liberal leanings and her talk show on the progressive channel CurrentTV would make her a tough sell to Republicans. In two terms in the Michigan statehouse, Granholm made clean energy a priority. She attracted solar, wind and electric vehicle companies to the state to replace lost automotive jobs and oversaw passage of a Renewable Portfolio Standard mandating that 10 percent of the state's electricity come from renewable sources by 2015.

Other possible candidates include Washington Gov. Christine Gregoire, who has promoted renewable and nuclear energy in her home state, and John Podesta, former chief of staff for President Clinton and CAP's current chair.

Weiss said it's also possible that Obama could select someone with business experience to head the department, a move that would signal the administration's intention to work with the electricity and industrial sector.

Several groups mentioned Duke Energy chairman, CEO and president Jim Rogers as a possibility, given his close ties to the White House. Rogers, who recently announced his intention to leave the Charlotte company in 2013, co-chaired the Democratic National Convention and has been a booster for Obama's renewable energy initiatives.

Whoever gets the job will have to deal effectively with Congress and the White House budget authors, who ultimately will determine how much money the department will have available to invest in renewables.

"I can't imagine an expansion of any kind of a loan guarantee program," said Loris of the Heritage Foundation. "Given our fiscal situation and some dissent about what the role of DOE is, I think that's an area that could be ripe for spending cuts."

David Foster, executive director of the BlueGreen Alliance, doesn't see the agency moving away from clean energy investments, given that clean energy has been "a powerful piece of the administration's success story."

But EDF's Keohane said there's plenty of room for the Energy Department to maneuver outside of the loan programs. The agency could continue to push energy efficiency standards for appliances or in the electric industry. It could also help craft rules and provide technical assistance as more states and localities eye smart grid technology.

If the loan program is slashed, Roy at the Center of Climate and Energy Solutions, expects the agency to focus on highlighting some of its successful loans and use them as a model to help make clean technology commercially feasible.    

"The point of investing in a solar or an electric vehicle plant is not to help one plant advance, it's to get the technology further down the learning curve and cost curve," Roy said.

The agency also could focus on R&D through its national labs and the popular Advanced Research Projects Agency-Energy (ARPA-E) program, which supports high-risk energy technology projects.

Under the right leadership, Roy said, the Energy Department can play an important role for the White House by promoting both its climate and its economic messages.

"Can we get back to a point where the administration is arguing about the importance of clean energy? That's going to drive things as much as anything," Roy said. "There's room to talk more robustly about the economic future and the role of clean energy. We need to see a return to that."