Sen. Rand Paul, R-Ky., proposed Wednesday an official U.S. declaration of war against the Islamic State group. Paul, a likely 2016 presidential contender, charged that President Barack Obama’s current operations targeting the terrorist group in Iraq and Syria are unconstitutional because he did not get authorization from Congress.

“I believe the president must come to Congress to begin a war and that Congress has a duty to act,” Paul said in a statement. He has not yet discussed the measure on the Senate floor, according to the Louisville Courier-Journal. The conservative leader called for a formal war declaration last month against the militant group also known as ISIS or ISIL. 

The U.S. Constitution grants Congress the sole power to wage war, but Congress has not formally declared war since 1941. Obama announced his campaign against the Sunni militant group in a primetime address in September, during which he asked for “support” from Congress, but he asserted, as all recent presidents have, that he had the right to act alone.

“I have the authority to address the threat from ISIL. But I believe we are strongest as a nation when the president and Congress work together,” Obama said in his address. “So I welcome congressional support for this effort in order to show the world that Americans are united in confronting this danger.”

At the time, few congressional leaders objected. “The administration has made an effort in recent days to brief members of the House and Senate on the range of options the president is contemplating,” House Speaker John Boehner, R-Ohio, said in a statement after Obama’s speech in September. “Those briefings and consultations will continue as members review his proposals, and I hope we can continue a dialogue about how to most effectively confront and destroy this enemy.”

White House Press Secretary Josh Earnest also asserted that the administration does not need a formal declaration from Congress to take military action. He said the administration can attack ISIS under via the Authorization for Use of Military Force that was passed by Congress shortly after Sept. 11, 2001. “We have been very clear that the president has all the authority he needs,” Earnest told reporters on Wednesday. Instead, he said the administration is seeking a new “authorization” from Congress to use military force.

But Paul insisted Obama is taking authority on powers that are exclusive to Congress. “Conservatives are mad at him about immigration. And they’re mad about him using executive authority on Obamacare,” Paul told the New York Times in November. “But this is another example where he doesn’t have much respect for Congress, and some conservatives don’t quite get that.”