Senate Democrats' legislation to reduce the U.S. deficit and increase borrowing authority would cut $2.2 trillion from deficits, about $500 billion less than Democrats claimed, the Congressional Budget Office said Wednesday.

The non-partisan budget scorekeeper for Congress also confirmed that about half of the Democrats' proposed savings would come from reduced costs as the U.S. withdraws from the wars in Iraq and Afghanistan.

Tuesday, the CBO said the Republican plan proposed by House of Representatives Speaker John Boehner would cut spending by $850 billion over 10 years, rather than the $1.2 trillion as the bill promised.

The CBO estimate has prompted Boehner to rework his legislation to identify more savings.

Overall, CBO said the Senate Democrats' bill would reduce spending by $1.8 trillion in programs Congress reviews annually and by $41 billion in benefit programs, such as payments to farmers. Another $375 billion in savings could accrue, CBO said, in lower interest costs on the debt because of lower annual budget deficits.

There are doubts whether the Republican or Democratic plans could gain enough bipartisan support in Congress to pass.

Congress is racing against an Aug. 2 deadline to pass deficit-reduction and debt limit legislation before the Treasury Department runs out of money to pay all its bills.