At the heart of the Biden administration’s efforts to advance clean energy is a $400 billion lending program that has backed dozens of projects across the nation, including battery factories in Ohio and Tennessee, the revival of a shuttered nuclear reactor in Michigan and a novel rooftop solar expansion in Puerto Rico.

Now, the Loan Programs Office at the Department of Energy is hustling to get money out the door before Donald J. Trump returns to the White House.

Mr. Trump hasn’t outlined his plans for the Energy Department. But some Republicans in Congress and advisers like Vivek Ramaswamy are already scrutinizing the loan office as they hunt for ways to slash federal spending. The conservative policy blueprint known as Project 2025 recommended that the loan office be “eliminated or reformed.”

Facing the prospect of drastic changes, the Biden administration has been working to finalize as many loans as possible before the changeover in January.

Under President Biden, the office has announced roughly $54 billion in loans or loan guarantees, which is still just a small portion of its lending authority. Of that, $19 billion was announced in the weeks after the election, including a conditional loan to help Rivian build an electric car factory in Georgia and a loan guarantee for an enormous power line in the Midwest.

Yet the office has closed only $13.5 billion of the deals to date — the rest are conditional commitments that could be delayed or halted by a new administration.