U.S. President Barack Obama addresses the country from the Oval Office on Dec. 6, 2015 in Washington, DC. President Obama is addressing the terrorism threat to the United States and the recent attack in San Bernardino, California. Saul Loeb-Pool/Getty Images

President Barack Obama urged Congress to declare war, in effect, on the Islamic State group Sunday night in a rare Oval Office speech in response to the San Bernardino, California, killings. Days after two suspected ISIS supporters opened fire on a party at a social services office, killing 14 people, Obama called on Congress to formally vote on the use of force against ISIS.

“The threat from terrorism is real, but we will overcome it,” Obama said during his 14-minute address. Using an acronym for the group, he declared: “We will destroy ISIL.”

It wasn't the first time Obama had asked Congress to go on record authorizing war against ISIS. But so far Republican lawmakers who control the legislative branch have refused to do so while at the same time critiquing Obama for being soft on terrorism.

The actual vote would be on a provision called the Authorization for the Use of Military Force, or AUMF. Federal law requires the president to notify Congress within 48 hours of ordering U.S. armed forces into a foreign military operation. The draft language put forth by the White House allows Obama "to use the Armed Forces of the United States as the president determines to be necessary and appropriate against ISIL or associated persons or forces."

A year ago, then-House Speaker John Boehner, R-Ohio, declared, “I would urge the president to submit a new Authorization for Use of Military Force regarding our efforts to defeat and to destroy ISIL.” Presidential candidates such as Sen. Ted Cruz, R-Texas, have also called for a war vote against ISIS, saying that “initiating new military hostilities in a sustained basis in Iraq obligates the president to go back to Congress and to make the case to seek congressional authorization,” while Sen. Rand Paul, R-Ky., has said, “I believe the president must come to Congress to begin a war and that Congress has a duty to act. Right now, this war is illegal until Congress acts pursuant to the Constitution and authorizes it.”

ISIS Targets Destroyed By American Air Strikes | FindTheBest

Obama began airstrikes in Iraq and Syria against ISIS last year and asked for war authorization in February while asserting that he had the legal right to wage war under a congressional resolution passed after Sept. 11, 2001. Obama has been operating on the AUMF’s from 2001 and 2002 passed under the George W. Bush administration as the legal justification for its airstrikes against ISIS.

Republican lawmakers have praised the war against ISIS while slamming Obama's methods. Sen. Marco Rubio, R-Fla., another presidential candidate, said it should read: “‘We authorize the president to defeat and destroy ISIL.’ Period."

Part of the problem is that Republican leaders haven't formed a united front on how to defeat ISIS, some pundits say. At the heart of the debate is whether the U.S. should send ground troops to fight ISIS and whether the war should be limited to ISIS territory in Syria and Iraq or extended to any nation where the militant group has supporters.

The Department of Defense has said defeating ISIS could take a minimum of three years.