Two top Republicans have warned of catastrophic consequences if Congress votes against striking Syria.

Senators John McCain and Lindsey Graham, Republican hawks who have long advocated deeper US involvement in Syria’s civil war, met President Barack Obama as part of his efforts to sell US politicians on military action.

McCain emerged from the meeting with a stern warning for fellow Republicans who may be considering voting against military action purely to damage the Democratic president.

“A vote against that resolution by Congress I think would be catastrophic,” said McCain.

“It would undermine the credibility of the United States of America and the president of the United States. None of us want that.”

Graham, who is facing a tough primary fight from his right for the Republican Senate nomination in South Carolina, warned of the wider consequences of a failure to back military action.

“I can’t sell another Iraq or Afghanistan, because I don’t want to,” Graham said.

“(What) I can sell to the people of South Carolina (is) that if we don’t get Syria right, Iran is surely going to take the signals that we don’t care about the nuclear program.

“It weighs on the president’s mind strongly about the signals we send,” Graham said.

The two senators also offered what appeared to be indications of evolving administration thinking about its Syria policy.

Obama has stressed that any US action, expected to include cruise missiles, would be “limited” and “narrow”.

But McCain suggested that more US muscle than expected might be deployed.

“We have been given some reason to believe that very serious strikes may take place as opposed to cosmetic,” McCain said.

“We now need to see a lot of the details.”