WASHINGTON (CNN) -- On the same day the top Democrats in Congress said a troop surge in Iraq would "stretch our military to the breaking point," Arizona GOP Sen. John McCain reiterated his support of increasing U.S. forces in the war-torn country and said they must be 'significant and sustained'.

"The worst of all worlds would be a small, short surge of U.S. forces," McCain said Friday during a speech at the American Enterprise Institute. "We tried small surges in the past and they've been ineffective because our commanders lack the forces necessary to hold territory after it was cleared."

In a letter to President Bush, Senate Majority Leader Harry Reid, D-Nevada, and House Speaker Nancy Pelosi, D-California wrote, "surging forces is a strategy that you have already tried and that has already failed. Like many current and former military leaders, we believe that trying again would be a serious mistake."

The president is expected to outline his new policy in Iraq during a prime-time address some time next week.