(CNN) -- President Barack Obama tried Tuesday to sell a military intervention he never wanted to an American public that opposes it, telling the nation that he needed authorization to attack Syria for chemical weapons use as leverage in a newly emerged diplomatic opening from Russia.
"Our ideals and principles as well as our national security are at stake in Syria," Obama said in making the case that the United States must act when dictators such as Syrian President Bashar al-Assad "brazenly" violate international treaties intended to protect humanity.
The 15-minute nationally televised speech initially was planned as Obama's final push to win support from a skeptical public and Congress for his planned attack on Syria for what his administration calls a major chemical weapons attack on August 21 that killed more than 1,400 people in suburban Damascus.
New offer impacts Obama's challenge
However, Monday's unexpected diplomatic overture by Russia changed the strategic and political equation. Under the Russian plan, which still lacks any details, Syria would turn over its chemical weapons stockpiles to international control.
That would meet Obama's main criterion of ending the chemical weapons threat by the al-Assad regime.
However, Russia canceled a U.N. Security Council meeting it had called for Tuesday and rejected an initial proposal by France for the framework of a resolution, raising questions about whether the diplomatic effort was serious or a stall tactic to put off a U.S. attack on Syria.
For Obama, the Russian proposal prompted by a seemingly off-the-cuff comment by U.S. Secretary of State John Kerry further muddied an already complex challenge in Syria compounded by public concerns of another possible military quagmire.
Obama called the Russian offer a potentially promising sign, but warned that "it's too early to tell whether this offer will succeed, and any agreement must verify that the Assad regime will keep its commitments."
Therefore, he said, he asked Congress to postpone a vote for now on authorizing military force against Syria.
In addition, the diplomatic push will provide more time for United Nations inspectors to report their findings on the August attack and allow his administration to continue rallying support for an international response, the president said.
At the same time, Obama said he ordered the U.S. military to remain ready to attack in order to "keep up the pressure and to be ready to respond if diplomacy fails."
Kerry made the same argument at a congressional hearing Tuesday, telling legislators that "nothing focuses the mind like the prospect of a hanging."
However, congressional support for military action reflected public opposition. A CNN/ORC International poll released Monday said 59% of respondents opposed congressional authorization of military action, while 72% said American strikes would achieve no significant goals.
Critics call the situation faced by Obama his own doing for a confused Syria policy that he has never fully explained.
"There's a degree of incoherence that I have never seen the likes of," veteran Republican Sen. John McCain of Arizona told CNN on Tuesday.
Obama's reluctance on Syria
For two years, Obama resisted calls by conservative hawks such McCain to back rebels fighting the al-Assad regime, saying the United States sought no role in the Syrian civil war.
When evidence of chemical weapons use emerged earlier this year, and Hezbollah fighters from Lebanon helped al-Assad's forces gain the upper hand, Obama agreed in June to provide military aid to the rebels.
The August attack clearly crossed a "red line" he declared earlier against chemical weapons use, prompting his decision for what he hoped would be an international military response against Syria.
However, Britain's Parliament voted against joining a military response, denying Obama a normally reliable ally. He then decided to seek authorization from Congress to provide political cover and buy time to build a broader international coalition.
Now legislators from both parties are threatening to oppose a resolution authorizing a military response, and Obama has asked for time to let the diplomatic process play out.
Kerry heads to Geneva on Thursday for talks with his Russian counterpart, who first offered his government's proposal Monday after Kerry earlier said Syria's turning over its chemical weapons was the only way to avoid a U.S. attack.
Syria agreed Tuesday to the Russian proposal, with Foreign Minister Walid Moallem saying his government was ready to disclose the location of its chemical weapons, halt production, and show its facilities to representatives of Russia, the United Nations, and other unspecified states.
At the same time, Russian President Vladimir Putin said Tuesday that "all of this will only mean anything if the United States and other nations supporting it tell us that they're giving up their plan to use force against Syria."
"You can't really ask Syria, or any other country, to disarm unilaterally while military action against it is being contemplated," Putin said in an interview with a Russian television network.
Obama: Threat of strikes still critical
For his part, Obama sought to persuade Americans of the opposite -- that the diplomatic stirrings by Russia and Syria occurred because of the credible threat of a military attack intended to deter Syria from using its chemical weapons again.
The president has not said whether he would launch strikes without the support of Congress.
Meanwhile, a bipartisan group of eight senators is working on an alternative resolution to the one authorizing military action already passed by the Senate Foreign Relations Committee. It would include "guidelines, reporting process and benchmarks that have to be met," McCain told CNN.
On the House side, Democratic Rep. Chris Van Hollen of Maryland said he was working on a version of a resolution that would authorize a military response if the diplomatic process failed to yield an acceptable result in 30 days.
The Obama administration has launched a sweeping lobbying effort, with the president meeting on Capitol Hill on Tuesday with senators from both parties as part of a series of classified briefings, hearings and other consultations on the Syria issue.
Since Friday, the administration has spoken with at least 93 senators and more than 350 members of the House, a White House official said Tuesday.
Despite such outreach, indications were that the congressional push wasn't working.
Sen. Mitch McConnell, the top-ranking Republican in the Senate, announced Tuesday he will vote against authorizing military action on Syria.
So did Democratic Sen. Ed Markey of Massachusetts, who won the seat vacated by Kerry when he became secretary of state.
A running CNN vote count showed most members of Congress remained undecided, with significant opposition in both chambers among the much smaller numbers who have announced their decision leaving the outcome in doubt.