Senator Tim Kaine, D-Va., delivered an urgent message to Congress on Thursday: It's been nine months since the U.S. began bombing the Islamic State group in Iraq and Syria and you still haven't done your job to authorize the use of military force, as the Constitution requires.

"The president does not have the legal power to maintain this war without Congress, and yet Congress, this Congress, the very body that is so quick to argue against Obama's use of executive power, even threatening him with lawsuits over immigration actions, is strangely silent and allows and executive war to go on undeclared, unapproved, undefined, and unchecked," Kaine said.

Kaine was joined by Sen. Jeff Flake on the Senate floor Thursday morning, where they reprimanded their fellow lawmakers for failing to do their job.

"We've deployed thousands of troops to Iraq and Syria...conducted more than 3,000 airstrikes...spent more than $2 billion of American taxpayer dollars... and lost the lives of service members," Kaine reminded the Senate.

"But here's what hasn't happened," he said. "Congress, whose most solemn power is the duty to declare war, has not done its job, has not debated this war, has not taken any formal step to authorize what was started unilaterally by the president nine months ago."

"I challenge my colleagues to take this seriously and promptly debate and pass an authorization for military action. We should have done this months ago," he said.

President Barack Obama first ordered airstrikes against the Islamic State group in Iraq and Syria on Aug. 8, 2014, doing so without the approval of Congress. Obama argues that he doesn't need Congress' approval, saying that the 2001 Authorization for the Use of Military Force (AUMF) justifies his actions.

The 2001 AUMF was passed by Congress after the Sept. 11, 2001 attacks, and gave former President Bush the authority to take actions against groups that perpetrated the attacks.

However, the Islamic State group, also referred to as ISIL or ISIS, "was not a perpetrator of the 9/11 attack," Kaine said, noting that the Islamic State group is not even an ally of al-Qaida.

"The only way the 2001 AUMF could possibly be stretched to cover ISIL is if we pretend that the authorization is a blank check giving the president the power to wage war against any terrorist group," Kaine said. "But Mr. President, that was precisely the power that Bush asked for in 2001, and Congress explicitly refused to grant that power even in the days after 9/11 attacks."

Another AUMF passed around the same time gave former President Bush the authority to topple Saddam Hussein's regime, but since that regime was ousted years ago, it no longer has any relevance, Kaine said.

"The War Powers Resolution of 1973 does grant the president some ability to initiate military action for 60 to 90 days prior to congressional approval, but also mandates that the president must cease military activity unless Congress approves it," Kaine said. "Here we've blown long past all the deadlines of the act, Congress has said nothing, and yet the war continues."

Obama finally sent an AUMF to Congress in February, saying he welcomes Congress' vote even though he said he didn't need it. But lawmakers were unable to reach an agreement and the war went on without any parameters on its duration or cost. Kaine pointed out how no AUMF has made it to either the House or Senate floor for debate.

"The silence of Congress in the midst of this war is cowardly and shameful. How could we explain to our troops, the public or ourselves the complete unwillingness of Congress to take up this important responsibility," he said, adding that it leaves the "impression that Congress is either indifferent about ISIL and the threat it poses, or lacks the backbone to do the job it is supposed to do."