Sen. McCain, who's been urging U.S. military action in Syria for almost two years, emerged from a White House meeting with fellow Republican hawk Lindsey Graham to give Obama's plan on Syria the thumbs-up, though he added he still had misgivings about the administration's strategy.
"We still have significant concerns but we believe there is in formulation a strategy to upgrade the opposition," McCain told reporters following his discussion with the president.
McCain, Obama's rival for the White House in 2008, has been a harsh critic of Obama for months on Syria. Earlier on Labour Day weekend, the Arizona lawmaker assailed Obama's announcement that he favoured action that would be "limited in duration and scope," saying in a statement with Graham that such a strategy would "send the wrong signal."
"We cannot in good conscience support isolated military strikes in Syria that are not part of an overall strategy that can change the momentum on the battlefield, achieve the president's stated goal of (President Bashar) Assad's removal from power, and bring an end to this conflict, which is a growing threat to our national security interests," the senators said.
On Saturday, Obama stunned the world — not to mention his team of national security advisers — when he opted against immediate action in Syria despite the administration's insistence that Assad had gassed his own people, killing almost 1,500 citizens in a Damascus suburb on Aug. 21.
Instead, Obama said, he would seek authorization from Congress, one of the most obstructionist and fractious in contemporary U.S. history.
McCain and Graham said they told the president that he must make a strong case for an attack if he has any hope of winning congressional support. Several of the senators' fellow Republicans are deadset against any action in Syria, particularly Tea Party darlings Rand Paul and Ted Cruz.
Legislators "must be assured that this is different from the past two years of neglect" on the part of the Obama administration, McCain said.
McCain warned, however, that striking now against Syria is far trickier since Assad is "moving his forces around." Talking to reporters outside the White House on Monday afternoon, both McCain and Graham also questioned the administration's wisdom in broadcasting the intention to strike.
McCain has been both a thorn in Obama's side and a boon in recent months.
He is the president's harshest Republican critic on foreign policy, but on domestic issues, the longtime senator has worked with Democrats and the White House on attempts to push through immigration reform and to slap down the Republican party's hard-nosed Tea Party caucus on budget and spending issues.
Obama is now hoping McCain can convince his fellow lawmakers to back a strike on Syria, but signs are emerging that the administration is facing an uphill battle in Congress and among a war-fatigued American public.
Paul, the libertarian Kentucky senator, plans to mount a lobbying effort in the Republican-controlled House of Representatives, where a growing bloc of members are wary and weary of war.
Secretary of State John Kerry told about 130 House Democrats on Monday that the U.S. must act to punish Assad, but was met with staunch resistance.
In a conference call, Kerry reportedly referred to Assad as a "two-bit dictator" who will "continue to act with impunity" if not brought to heel. He was met with skepticism from some Democrats, including Minnesota congressman Rick Nolan, who compared Syria to Vietnam.
"After a three-hour classified briefing, and taking time to read all the classified documents, what I have heard and read has only served to convince me more than ever of the folly and danger of getting America involved in the Syrian civil war," Nolan said in a statement following the call.
"I will vote and work against President Obama's request for open-ended authority to launch military strikes against the Syrian army."