Maryland Sen. Ben Cardin, the top Democrat on the Senate Foreign Relations Committee, said Trump would need a war authorization
to bomb Syrian President Bashar al-Assad's regime.
Like the Obama administration, the Trump administration has launched airstrikes against ISIS using legal justification from the 2001 war authorization that passed after the September 11, 2001 terrorist attacks. But that would not cover Assad, Cardin argued.
"If we decide to do military in Syria, he really needs to come to Congress," Cardin said. "In regards to an attack against the Assad government, there is no authorization."
Connecticut Sen. Chris Murphy, a Democrat who has pushed for a war authorization against ISIS, also argued Trump would not have the authority to attack the Assad regime.
"Dropping bombs inside a civil war was a bad idea in 2013, it's a worse idea in 2017," Murphy said. "It will make some Americans feel better, but it will make that battle space more chaotic and end up with more people getting killed, not less. And again, he doesn't have the authorization from Congress to do this."
Rep. Jackie Speier, a Democratic member of the House Armed Services Committee, told CNN's Erin Burnett that Congress should evaluate and decide on a war in Syria before Trump moved in on his own.
"I think if we are going to put boots on the ground, we have to authorize the use of force," Speier said. "That should be something debated by the Armed Services Committee and by the full House. We must engage in wars first by evaluating them and Congress taking action, not by allowing the President to act independently, which has been going on now for two decades."
When the Assad regime violated President Barack Obama's "red line" in 2013
with a chemical attack, the Obama administration prepared to strike Assad, before the President decided to go to Congress for authorization first.
Congress failed to pass the authorization
and the US did not strike Assad, which many Republicans -- including Trump -- say was a decision that emboldened the Syrian leader to take further heinous actions like this week's chemical attack. Republican Sens. John McCain and Lindsey Graham on Thursday called for grounding Assad's air force in response to this week's attack.
Senate Foreign Relations Chairman Bob Corker, a Tennessee Republican, said in the short-term, the President has the authority to take action without getting congressional approval first.
"The prudent course of action would be for him to go to Congress and consult, discuss privately what he is going to do," Corker said. "I'm sure they will take that course of action, or at least I hope they would. So, they have the authority without that, but my sense is that they will come to us."
When Obama tried to get authorization to strike Assad, Republicans were reluctant to get on board to vote for it, Cardin said.
This time, he added, there could be an easier path, especially if it's tailored specifically to respond to the chemical attack.
"Certainly if it's well tailored toward the chemical weapons violations, you might be able to do it," he said.
It's not just Democrats who want Trump to come to Congress. Sen. Rand Paul, known for his libertarian ideology, told Fox News Radio, "Short of Congress voting on it, I'm opposed to illegal and unconstitutional wars."
"The first thing we ought to do is probably obey the Constitution," Paul said on "Kilmeade and Friends."
At the same time, the situation on the ground in Syria is more complicated for Trump now than it was back in 2013, as Russia has gotten involved militarily to bolster the Assad regime.
Maine Sen. Angus King, an independent who caucuses with the Democrats, said he was most concerned about a large US ground force deploying to Syria, but noted that Russia makes even trying to ground Syria's air force difficult.
"It gets complicated because it's not only Assad's air force, it's Russia's air force," King said.