Democrats in a Do-Si-Do Over Negative Campaigning

Over the course of 2007, the campaign paid more than $20,000 to Native Energy, a firm that supports renewable-energy projects and enables companies to balance travel and office carbon emissions.

The move was well received by environmental groups. Only John Edwards had taken a similar step. Barack Obama's campaign pays a special fee to its charter air firm to offset emissions from its jet travel, but it does not offset its full carbon footprint.

But earlier this year, as the Clinton campaign's cash dried up, the campaign had to place its desire to remain carbon-neutral lower on the priority list. Reports filed to the Federal Election Commission for January and February show that the campaign's debt to Native Energy piled up until it stood at $11,834 at February's end.

Blake Zeff, a Clinton campaign spokesman, said the campaign has not wavered in its commitment to the environment. The campaign paid $6,338.52 to the firm March 13, catching up on its debt through January, he said.

Native Energy spokesman Billy Connelly said he didn't have the payment data readily available but he thinks the Clinton campaign is "doing a thorough job."

"They're taking this very seriously," Connelly said.

-- Matthew Mosk