A Democratic senator is planning to introduce legislation authorizing U.S. airstrikes against the Islamic State in Syria, in a bid to ensure President Obama is not waiting on Congress to escalate the military campaign.

When Congress returns next week, Sen. Bill Nelson, D-Fla., says he plans to introduce a bill to "ensure there's no question that the president has the legal authority he needs to use airstrikes in Syria."

Nelson said in a statement: "Let there be no doubt, we must go after ISIS right away because the U.S. is the only one that can put together a coalition to stop this group that's intent on barbaric cruelty."

Legal analysts and lawmakers on both sides of the aisle have long been at odds over what type of military action requires congressional approval -- the debate flared during the 2011 operation in Libya, and amid the threats of military action in Syria last year -- and this case is no different.

Nelson told Fox News on Wednesday that he doesn't personally think the president needs legislative approval for the expanded military action. But he said his legislation would "get rid of" any "ambiguity" for the president. "The head of the snake is in Syria," Nelson said.

The president also has taken heat in recent days for what critics describe as a muddy approach to the Islamic State. Obama admitted last week his team does not have a strategy yet for tackling the group, also known as ISIS, in Syria.

He tried to clarify those remarks during a press conference on Wednesday -- on the heels of another American journalist's execution at ISIS hands -- and suggested the hang-up might be congressional authorization.

"I was specifically referring to the possibility of the military strategy inside of Syria that might require congressional approval," Obama said.

The question of whether Congress needs to weigh in bubbled during the August recess, as lawmakers claimed the issue should be put to a vote. Sen. Tim Kaine, D-Va., Sen. Rand Paul, R-Ky., and others said Congress should play a role.

After Nelson announced his legislation, Rep. Frank Wolf, R-Va., on Wednesday announced plans to introduce a similar bill.

But whether the president even needs to seek a vote for such military action is an unsettled issue.

The White House and Congress, during any military hostilities, navigate a web of laws and powers deriving from the Constitution, and U.S. laws and policies dating back decades.

One of those is the 1973 War Powers Resolution. The resolution, passed in defiance of then-President Richard Nixon at the end of the Vietnam War, says presidents must seek congressional approval to keep U.S. forces in hostilities for more than two months.

During the Libya operation, the White House argued that the military action did not rise to the level of “hostilities,” while continuing to keep Congress formally apprised.

The White House is doing the same this time, sending Congress formal, written updates on airstrikes in northern Iraq, which it says is “consistent with the War Powers Resolution.”

But whether the White House would seek formal congressional approval under that resolution is unclear. Presidents of both parties have defied the 60-day limit in that resolution, ever since its passage.

Another factor is the 2001 authorization for use of force after the 9/11 attack, and the 2002 authorization for use of force in Iraq. But lawmakers have questioned whether either of those authorizations applies here – one complication is that the 2001 measure was against Al Qaeda, and the Islamic State is not technically Al Qaeda.

Still, other legal scholars have pointed simply to the Constitution as the only document that matters here.

John Yoo, who was a major figure in the George W. Bush administration’s Justice Department, has argued that despite the provision giving Congress the power to “declare war,” that is not the same as hostilities.

During the debate over military action in Syria last year, he argued that throughout history, “neither presidents nor Congresses have acted under the belief that the Constitution requires a declaration of war before the U.S. can conduct military hostilities abroad.”

He noted that the U.S. only formally declared war five times, while Congress gave “authorization” for using force a handful of other times, including in 2001 in Afghanistan.

But in between those major military actions were more than 100 engagements where military force was used, often against groups – “Indians, Barbary pirates and Russian revolutionaries” – as opposed to enemy nations.

Yoo argues that Congress’ “check on the presidency” is, rather, its ability to cut funding for operations it opposes.

Asked Wednesday whether Obama needs further congressional support in the case of ISIS, Yoo told FoxNews.com in an email: "I think that the President has both constitutional and statutory authority."