Rewriting or doing away with those standing rules of war on terrorism, known on Capitol Hill as the Authorization on the Use of Military Force (AUMF), will do more harm than good in the fight against al Qaeda, Forbes said.
"I think it could end up worse than it is now," he told reporters during a breakfast round table in Washington on Tuesday.
Forbes's concern is that counterterrorism missions could become so watered down, as a result of the proposed changes, their effect to deter terror groups would be minimal, if not nonexistent.
Michael Sheehan, head of special operations and low-intensity conflicts at the Pentagon, told Congress in May he sees no need to change the current rules of war.
“At this point, we're comfortable with the AUMF as it is currently structured. Right now it does not inhibit us from prosecuting the war against al Qaeda and its affiliates,” he told the Senate Armed Services Committee in May.
Forbes also criticized the White House for "talking too much and classifying too much" on its ongoing counterterrorism operations.
"You either use [counterterrorism missions], or you stay quiet," he added, referring to the multiple leaks of classified information on sensitive missions, such as the Navy SEAL assault that killed al Qaeda leader Osama bin Laden in 2011.
His comments come as House Defense lawmakers prepare to fold in a number of changes to the current counterterrorism rules in their version of the Department of Defense's (DOD) fiscal 2014 budget bill.
The full committee is set to mark up that FY 2014 spending legislation on Wednesday.
One of the two proposals in the House bill will require President Obama to notify the congressional Defense committees each time a so-called "kill/capture" operation, such as armed drone strikes, is launched against suspected terror targets.
The other proposal will force the Pentagon and White House to review all groups or individuals now characterized as “associated forces” under the 9/11 counterterrorism rules.
Both measures are reportedly receiving bipartisan support on the Defense committee and support the administration's efforts to scale back its aggressive counterterrorism strategy.
“I look forward to engaging Congress and the American people in efforts to refine, and ultimately repeal, the AUMF’s mandate,” Obama said in an address at the National Defense University in May.
Obama argued that unless the 12-year-old rules are changed, Congress risked giving future presidents unbound powers.
“Our systematic effort to dismantle terrorist organizations must continue. But this war, like all wars, must end. That’s what history advises. That’s what our democracy demands,” Obama said at the time.
Forbes admitted the nature of the war against al Qaeda has changed dramatically in the decade since the AUMF was passed.
Rule changes to increase congressional oversight of counterterrorism missions are necessary, he said. But repeal of those counterterror rules would leave the United States shorthanded in the war against al Qaeda that is still ongoing.
"You have to put that stuff on the table ... [because] every war evolves," the Virginia Republican said. "But, I think we all know that this war is not over."