The White House is battling for congressional approval to bomb Syria, as two top Republicans warned that a "no" vote after President Barack Obama had threatened action would be catastrophic.
Senators John McCain and Lindsey Graham emerged from talks with Obama suggesting that the White House could be mulling a wider military campaign in Syria than first thought, along with more support for the opposition.
Obama shocked Washington when he decided to seek support for military action in Syria from Congress, when it seemed US cruise missile strikes on President Bashar al-Assad's forces and assets were imminent.
McCain and Graham appeared to offer qualified backing for Obama's plans.
"A vote against that resolution by Congress I think would be catastrophic," said McCain.
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"It would undermine the credibility of the United States of America and the president of the United States. None of us want that."
Graham warned of the wider consequences of a failure to back military action.
The senators also hinted at the administration's evolving strategy for Syria.
Obama has stressed that any US action, expected to include cruise missile attacks, would be "limited" and "narrow".
But McCain said he had "been given some reason to believe that very serious strikes may take place as opposed to cosmetic (ones)".
"I don't think it is an accident that the aircraft carrier is being moved over in the region," he said.
ABC News reported Monday that the USS Nimitz aircraft carrier was moving westward toward the Red Sea, though had not yet received orders to support a strike on Syria.
Obama's plan could be to degrade President Bashar al-Assad's capabilities and upgrade those of vetted opposition groups, McCain suggested.
Graham indicated that the administration, which resisted arming Syrian rebels for months, may be prepared to stiffen a nascent plan - announced after previous, small scale chemical weapons attacks - to increase military aid to some rebel groups.
"There seems to be emerging from this administration a pretty solid plan to upgrade the opposition," Graham said, saying regional players like Saudi Arabia, Turkey and Jordan should play a key role.
Secretary of State John Kerry and Defence Secretary Chuck Hagel will testify to the Senate Foreign Relations committee on Tuesday in what will be one of the most high profile political set pieces in Washington in weeks.
