But the Senate proposal is being crafted in case those talks fail and to provide an alternate route to military intervention if Syria balks.
The proposal is largely supported by those, like Casey, who have backed military strikes, who hope a new approach might win over colleagues, and who believe the threat of military action is a critical tool for Obama in negotiations.
Syria and Russia would never have agreed to talks, Casey said, "if there was any doubt about our ability to use force."
The "threshold question" on chemical weapons, he told The Inquirer, is, "Does the international community condemn them and walk away, or do you take an action?"
But having the Syrian regime turn over its chemical weapons without the United States' having to resort to military action could be an "even better result," Casey said,
Obama and those advocating for military intervention needed a new option after the president's initial push for action faltered in the face of widespread opposition from the public and lawmakers, and faced near-certain defeat in the House.
With Obama turning to diplomatic efforts, expected votes on military action were called off.
As those negotiations begin, a bipartisan group brought together by Sen. John McCain (R., Ariz.) has spent the last few days working on plans to adopt the Russian proposal, giving Syria a brief window - "days and not weeks," Casey said - to turn over its chemical weapons, eliminating the primary concern that brought on Obama's push for military intervention.
The group began with eight senators but is said to have grown.
Casey stressed that any resolution should include military action if Syria does not follow through.
"I'm not going to support something that takes the credible use of force off the table," he said.
Sen. Robert Menendez, the New Jersey Democrat who is chairman of the Foreign Relations Committee, is also close to the talks, having led hearings on the issue and sponsoring the resolution authorizing military attacks.
His approval will be critical. But he sounded skeptical of the new idea Tuesday.
"I'm skeptical but hopeful," Menendez told the BBC. "This has to be tested. It cannot be just an effort to buy time. It has to be a serious effort."
For Casey, who until recently chaired a subcommittee that oversaw U.S. affairs in Syria and other parts of the Middle East, the debate on intervention has brought his quiet work on the issue into focus at a critical moment. (Casey left the foreign affairs panel this summer for a spot on the Finance Committee.)
He was an early and vocal supporter of Obama's call for military strikes - as was Menendez - but as opposition mounted, the president and his allies searched for another option.
Casey said conversations about an alternative began last weekend and gained steam Monday night off the Senate floor, where a clutch of senators met. There were meetings and calls Tuesday, with more expected this week.
Many question whether Syria and Russia can be trusted as negotiating partners.
"If I had to make a determination right now, 'Do you trust the Russian Federation in this?' I would have to say no," Casey said. "But I'm willing to hear more, and maybe the trust will grow."
McCain and others also expressed skepticism about diplomatic solutions, particularly when dealing with Russia and Syria.
One indication of the potential pitfalls came when Russian President Vladimir Putin said Tuesday that any deal would hinge on the United States' renouncing the use of force - an idea Casey and others immediately rejected.
But a direct path to a military strike - without talks - appears blocked.
U.S. Rep. Pat Meehan, a Republican from Delaware County, became the latest lawmaker from the Philadelphia area to come out with a firm "no" on authorizing the use of U.S. force.
"It is not clear how intervening in Syria's civil war, absent an imminent threat to the United States, best serves our national interests," Meehan said in a news release. "Rather, its plans have the potential to open a Pandora's box of regional conflict that imperils America's critical allies, particularly Israel.
"Under these circumstances, I cannot support the president's request."
Contact Jonathan Tamari at email@example.com or follow on Twitter @JonathanTamari. Read his blog "Capitol Inq" at www.inquirer.com/capitolinq.