Illinois’ Democratic U.S. senator is breaking with the White House on whether Congress must sign off on military action in Libya.

Senator Dick Durbin, usually a staunch supporter of the White House, said Friday that he isn’t buying the argument that President Obama can launch military airstrikes in Libya without congressional approval.

“Congress alone has the constitutional authority and responsibility to declare war,” Durbin told reporters at an unrelated press event.

His comments come after the White House this week released a report to Congress arguing that the Obama administration does not need lawmakers’ stamp of approval in order to continue airstrikes with U.S. warplanes and unmanned drones in Libya. The battle between the branches has been brewing since March, when Mr. Obama authorized the attacks to weaken the regime of Libyan leader Moammar Qaddafi and prevent civilian casualties during an uprising in Libya.

But some Republicans and anti-war Democrats say the president should have sought congressional permission. In the report released Wednesday, the White House maintains that U.S. forces are not facing the same “hostilities” as they would in a conventional war. The U.S. is now launching airstrikes in a supporting role in the NATO-led mission in Libya, the report says.

But Durbin, the No. 2 Democrat in the Senate, said he disagreed with the White House’s reading of the War Powers Act.

“The fact that we haven’t had any Americans killed - no planes shot down - is not, from my point of view, what should determine the constitutional question,” Durbin said, adding that he would vote to authorize U.S. action in Libya if it comes before Congress.

The White House is facing incredulity from some GOP lawmakers, such as Sen. John McCain of Arizona, and House Speaker John Boehner, who say the Obama administration’s arguments don’t reconcile with the reality of U.S. warplanes flying combat missions in a foreign nation. Boehner has even threatened to try and limit funding for Libyan operations in the House.