More than two dozen Senate Democrats are pressing a federal housing regulator to provide a new analysis of the effects of mortgage principal reductions on taxpayers for loans backed by Fannie Mae and Freddie Mac.

Sens. Robert Menendez (D-N.J.), Jack Reed (D-R.I.), Mark Warner (D-Va.) and Jeff Merkley (D-Ore.) led a group of 30 senators on Wednesday in calling on Edward DeMarco, acting director of the Federal Housing Finance Agency (FHFA), to provide Congress with an updated and "accurate" analysis of a recent study they say is flawed.

"We believe that the analysis has several critical flaws and also has not been updated for new developments," they wrote in a letter to DeMarco.

“We respectfully request that FHFA update its initial study, refining the earlier analysis and including new developments, especially the administration’s tripling of incentives for principal reduction," they wrote.

DeMarco argued on Wednesday that Fannie and Freddie are playing a leading role in providing assistance to homeowners.

"It has been well-publicized that there is one form of loan modification that FHFA has not embraced, that being principal forgiveness," he said during a speech before the Boston Security Analysts Society.

"To be clear, the disagreement is not about helping borrowers," he said.

"FHFA, with the Enterprises, has been making great efforts to assist troubled homeowners with underwater mortgages who have the ability to make a mortgage payment and a willingness to do so."

DeMarco said the FHFA is evaluating the recent Treasury changes to the Home Affordable Modification Program (HAMP) regarding principal forgiveness and should have a decision this month.

Up until now, Fannie and Freddie have modified 1.1 million loans since the final quarter of 2008 under HAMP.

He explained that the goal is to reduce a borrower’s monthly payment to an affordable level and for those underwater on their loans, that has been achieved by principal forbearance, charging a zero rate of interest on the forbearance amount and deferring its repayment.

"If the borrower remains successful in this modified loan, this approach preserves for taxpayers an ultimate recovery on the debt," DeMarco said.

"Stated differently, the principal forbearance mod being used by Fannie Mae and Freddie Mac produces the same, lower monthly payment as a modification based on principal forgiveness," he said.

"These borrowers are demonstrating a continued willingness to meet their mortgage obligations. This should be recognized and encouraged, not dampened with incentives for people to not continue paying. "

The lawmakers suggested that recent testimony before a Senate panel should be taken into consideration in an updated study. Witnesses during that hearing said the FHFA’s original analysis failed to consider whether Fannie and Freddie loans had mortgage insurance or not, failed to use real-world principal reduction data and had a variety of other technical flaws that could have changed the outcome of FHFA’s analysis.

"We believe that FHFA must be fully transparent with this new analysis and look at targeted solutions for borrowers in different situations, rather than the 'all or nothing' approach that was used in the previous analysis," they wrote.

They are asking for a new report within 30 days, saying in the letter that it is “not only part of your responsibility as conservator to conserve taxpayer assets, but also part of your statutory responsibility to maximize assistance for homeowners to minimize foreclosures.”