Two top House Republicans are questioning whether the Environmental Protection Agency effectively cooked the books to minimize the cost estimates of the Democratic plan to cap carbon emissions.

Rep. Darrell Issa (R-Calif), ranking member of the House Oversight and Government Reform Committee, and Rep. Jim Sensenbrenner (R-Wis.), ranking member of the Energy Independence and Global Warming Committee, say the EPA used a 2.5 percent growth rate for the gross domestic product instead of the 3.3 percent rate used by President Barack Obama in his budget. The lawmakers say that the difference amounts to $1.2 trillion discrepancy and that they are demanding an explanation in a letter to EPA Administrator Lisa Jackson.

“In EPA’s analysis, the agency assumed U.S. GDP growth for 2010 to 2019 at a rate of 2.5% per year,— the lawmakers wrote. “In contrast, President Obama’s budget proposal, released just 2 months ago, relied on assumed GDP growth of 3.3% for the same period … the discrepancy is significant.—

A lower GDP number would reduce calculations for greenhouse gases and compliance costs, the Republicans said. They also charged that the EPA’s analysis relied on offset numbers that were incorrect.

“Given the speed with which the [energy bill] is expected to move through the House, we would appreciate a written response to this inquiry no later than Friday, May 8, 2009,— the letter states. “Additionally, we would like to arrange a briefing with our staff to clarify these issues in the analysis.—

EPA spokeswoman Adora Andy defended the disparity in the figures. The EPA estimate was based on a 600-page bill, while the White House’s number was based on a concept, she said.

The White House based its GDP growth rate on the 2008 figures from the Energy Information Administration’s Annual Energy Outlook because 2009 numbers were not available, Andy said. Weeks later, when the EPA began analyzing the Democratic energy bill, the 2009 numbers were released.

“The administration is committed to bringing the best-evaluated information to the Members of Congress working on this historic legislation,— Andy said.

House Republicans charged this week that the cap-and-trade proposal that would put a cap on carbon emissions and allow companies to trade pollution levels would end up raising energy costs significantly across the country.

“This discrepancy is indicative of what may be a disturbing trend by the administration to manipulate facts and figures to justify their cap-and-tax bill,— a GOP aide said. “The questions raised in this letter call into question the administration’s credibility, and it would not be at all surprising if other discrepancies are uncovered as we continue to do a deeper dive into the Waxman-Markey bill.—