The president is hoping his old 2008 rival will help him convince Congress that Syria needs to pay a price for using chemical weapons in an attack in the Damascus suburbs that killed at least 1,429 civilians.
Mr. Obama is calling for a limited strike but both senators want more.
In a joint statement, they said: "We cannot in good conscience support isolated military strikes in Syria that re not part of an overall strategy that can change the momentum on the battlefield."
Still, Mr. Obama hopes the two senators can help him in his uphill battle and convince skeptical lawmakers to fact.
This as the president did somewhat of an about-face this Saturday deciding to seek congressional approval for a military strike against the Assad regime in Syria.
As Congress debates, the Arab League of Foreign Ministers is calling on the United Nations and the international community to "take deterrent measures to stop the Syrian regime." They stopped short of fully endorsing military action.
In a rare interview, Syrian Presdient Bashar Assad spoke out about the possibility of a U.S. strike.
He told a French paper: "Nobody knows what will happen (after such strikes). Everyone will lose control of the situation when the powder keg explodes. Chaos and extremism will spread. The risk of regional war exists."
Syria's allies have sent a strong message of their own to America about taking military action.
"We hope that both the government and Congress in the U.S. would consider the case very carefully and would consider the consequences of any military intervention in Syria," said Abbas Araghchi, Iran's Deputy Foreign Minister. "The consequences are disastrous and it would not remain inside Syria, it would spread to the whole region."
Meanwhile, the U.N. is trying to expedite the analysis of tests from samples its team collected from the sites of the alleged chemical weapons attack.
Secretary of State John Kerry made the case on five Sunday talk shows predicting Congress will give the president a green light but leaving it unclear what he will do if they don't.
With Navy ships on standby in the eastern Mediterranean ready to launch missiles, Congress on Sunday began a series of meetings that are expected to continue over the next several days in preparation for a vote once lawmakers return from summer break, which is scheduled to end Sept. 9.
About 70 lawmakers convened on Capitol Hill in a secure auditorium to hear directly from the president's national security team presenting evidence against Syria.
"The consequences of the Congress of the United States overriding a decision of the president of the United States of this magnitude are really very, very serious," said Sen. John McCain (R-Arizona).
On Sunday, Kerry said the administration learned of the sarin use through samples of hair and blood provided to Washington from first responders in Damascus.
"We know that the regime ordered this attack," he said. "We know they prepared for it. We know where the rockets came from. We know where they landed. We know the damage that was done afterwards."
A carrier group is also moving west, closer to the Red Sea. A senior defense official says the movie is prudent positioning and not a specific mission.
The Associated Press contributed to this report.
