US President Barack Obama received a big boost in his efforts to get Congressional authorisation for a military strike against Syria on Tuesday. A top Republican senator, John McCain, supported the move saying it would be “catastrophic” to reject such an action.

“A vote against this would be catastrophic in its consequences, not only as far as this issue is concerned, but in the future,” McCain told reporters at the White House after his meeting with Obama. He was joined by another top Republican senator Lindsey Graham.

During the meeting at the Oval Office, which lasted for about an hour, Obama underscored that America is stronger when the President and Congress work together to stand up for national interests, a senior White House official said.

“The President made clear his view that the failure to take limited action against Assad would unravel the deterrent impact of the international norm against chemical weapons use, would endanger US allies in the region and would risk emboldening Assad and his allies, Hezbollah and Iran,” the official said.

Obama’s meetings with top Republican senators came ahead of secretary of state John Kerry and defence secretary Chuck Hagel’s visit to the Capitol Hill on Wednesday to make the case to Congress on why targeted military action is necessary to hold the Assad regime accountable for their alleged use of chemical weapons, a state department official said.

Briefing reporters after their meeting with Obama, the two senators favoured passing a resolution to authorise for a military action against Syria. “We want to work to make that resolution something that the majority of the members of both houses can support,” said McCain, a former Republican presidential candidate.

“We do want an articulation of a goal that over time will degrade Bashar Assad’s capabilities, increase and upgrade the capabilities of the free Syrian Army and the free Syrian government so that they can reverse the momentum on the battlefield, that is presently not in their favour because they have not received the assistance that they need, while Bashar Assad has received an abundance of capabilities from his sponsors, Russia and Iran,” McCain said.

“The first thing I suggested to the President is (to) give the Opposition a chance to speak directly to the American people. John and I and the President all believe that the Syrians by nature are not Al-Qaeda sympathisers. They’re not trying to replace one dictator, Assad, who has been brutal, his whole family has been brutal for generations, to only have Al-Qaeda run Syria. That makes no sense,” Graham said.

“But it’s time for the Syrian Opposition to step forward. I want a statement from the Syrian Opposition that if we get in charge of Syria with your help, we’re going to renounce chemical weapons and in the new Syria there will be no chemical weapons because we’re going to turn them over to the international community,” Graham said.

The Pentagon said it has been an imperative of the US to engage in strong diplomatic efforts with the Syrian Opposition, to talk to them on a regular basis.

“We need to identify moderate Opposition groups that can come together and become more cohesive and oppose the regime of Bashar al-Assad,” Pentagon press secretary George Little told CNN.

“We’re not blind to the fact that there are terrorist elements in Syria. But we believe that at the end of the day the Syrian people will reject extremism and that they will adopt a diplomatic process that gets them to a new political landscape in their own country, that does not involve Bashar al-Assad,” George Little said.

In a letter, Congressman Frank Wolf called on Obama to immediately set up an advisory panel made up of former secretaries of state, ambassadors, statesmen and senior military officers to assist his administration in dealing with the situation in Syria.

Wolf said former secretaries of state Jim Baker and Madeline Albright, former ambassadors Edward Djerejian, Theodore Kattouf and Ryan Crocker and Generals John Abizaid, Anthony Zinny and David Barno would all be good candidates to serve on what he calls the Syria Advisory Group.

Meanwhile, the state department official said secretary Kerry will argue that the failure to take action against Assad unravels the deterrent impact of the international norm against chemical weapons use and endangers our friends and our partners along Syria’s borders, including Israel.

“Kerry will underscore that anyone who is concerned about serious issues of nonproliferation involving Iran and North Korea should support this action lest we send a message to those regimes that America’s statements of consequences are without meaning,” the official said.

“What you’ll hear from Hagel is a very strong endorsement of the view that chemical weapons use in Syria, and elsewhere for that matter, violates basic standards of human dignity, and also it’s in our national interests to prevent their use,” Little told the CNN.

“Think about the precedent this could set, not just in Syria for the Assad regime, but for Iran, for Hezbollah, for North Korea and for others. We’re deeply concerned about the precedent that if we don’t act, that others may believe that they might be able to use chemical weapons against their own innocent populations in the future,” Little argued.