WASHINGTON — The world can’t ignore the “horror” of Syria’s chemical weapons attack, Senate Foreign Relations Committee Chairman Bob Menendez said Tuesday at the start of a hearing on President Obama’s bid for congressional authorization of military action against Syria.

“This decision will be among the most difficult any of us will be asked to make,” said Menendez, D-N.J. Senators need to “put aside political differences and personal ideologies, forget partisanship and preoccupations, forget the polls, politics and personal consequences.”

There is no doubt the regime of Syrian President Bashar Assad is responsible for the attack, Secretary of State John Kerry told panel members. “It did happen and the Assad regime did it,” Kerry said as he appeared with Secretary of Defense Chuck Hagel in the committee’s first hearing on using force in Syria.

Comparing Assad to Adolf Hitler and former Iraqi leader Saddam Hussein, Kerry said, “History holds nothing but infamy for those criminals and history also holds very little sympathy for their enablers.”

Obama began his full-court press Tuesday with lawmakers, calling for a “prompt” vote on an issue he has tied to U.S. moral authority and global leadership. The White House is making its case in meetings with lawmakers and in congressional hearings.

Sen. Bob Corker, the top Republican on the Foreign Relations panel, indicated a potential hurdle to authorizing military force.

“The implementation of this is very, very important and I think there’ve been mixed signals about what that implementation is actually going to mean and the effect it’s going to have on the country,” Corker said.

Corker also raised concerns that the administration isn’t doing more to help the Syrian opposition.

“I am still totally dismayed by the support we are giving to the vetted, moderate opposition,” he said, citing administration promises. “Very little of that has occurred.”

Congressional leaders of both parties expressed clearer backing Tuesday for Obama’s request.

“The use of chemical weapons is a barbarous act,” House Speaker John Boehner, an Ohio Republican, told reporters after he and other congressional leaders consulted with Obama Tuesday morning at the White House. Only the United States has the capability to stop Assad and “to warn others around the world that this type of behavior is not going to be tolerated,” the speaker said.

The administration dispatched officials to Congress to give senators a closed-door classified briefing on the Aug. 21 attack in a Damascus suburb that killed more than 1,400 people, more than 400 of them children.

Obama said Tuesday that he’s open to changes in the resolution authorizing force against Syria to respond to concerns, and would let lawmakers redraft authorization language the White House had sent to Congress.

Corker told reporters earlier that the Senate probably will act before the House on a resolution to use force in Syria. The Tennessee Republican said he’s close to agreement with Menendez over how a more limited resolution would be structured.

House Majority Leader Eric Cantor of Virginia echoed Boehner’s support, saying in a statement that “America has a compelling national security interest to prevent and respond to the use of weapons of mass destruction, especially by a terrorist state such as Syria.”

House Minority Leader Nancy Pelosi, a California Democrat, said “the evidence on the intelligence is clear” and Congress probably will support Obama’s bid for approval of action. The chairmen and ranking members of national security committees also were among those at the White House meeting.

Speaking at the White House earlier Tuesday, Obama vowed that any strike by the U.S. will be limited and “proportional,” and won’t involve U.S. ground troops. He indicated he’s open to changes in the resolution authorizing force to respond to concerns of lawmakers.

“This is not Iraq, and this is not Afghanistan,” Obama said at the White House.