"There are times where we have to make hard choices if we're going to stand up for the things that we care about," he said. "And I believe that this is one of those times."
Two recent polls show Americans oppose airstrikes, with a Pew Research Center survey showing 48 percent opposed to 29 percent in favor and a Washington Post-ABC News poll showing 59 percent opposed and 36 in support. Both surveys were taken over the recent Labor Day holiday weekend as the U.S. released its assessment of whether the Syrian government used chemical weapons and Obama announced he would seek congressional approval.
Sen. Dianne Feinstein, D-Calif., chairwoman of the Senate Intelligence Committee, said the public sentiment might be different if Americans could see the evidence from the chemical weapons attack, including the convulsions and other side effects of the nerve gases.
"They don't know what I know. They haven't heard what I've heard," she said.
An Associated Press survey found 34 senators in support or leaning in favor of authorizing military action, 32 against or leaning that way and 34 undecided ahead of votes next week. Tallies in the House show a significant number of Republicans and Democrats are also opposed to military action or leaning against it.
Senate Majority Leader Harry Reid on Friday formally introduced the resolution, which would authorize the "limited and specified use" of the U.S. armed forces against Syria for 90 days while prohibiting American ground troops from combat. Lawmakers return from their five-week recess on Monday and will begin to debate, with a Senate vote to move ahead on the resolution expected Wednesday.
"I think we're going to get 60 votes. It's a work in progress," Reid said.
Obama's unexpected decision last week to seek congressional approval halted what had seemed to be a march toward quick military action in retaliation for a chemical weapons attack the U.S. says was perpetrated by Assad's government. Obama has repeatedly said the deployment of the deadly gases would cross a "red line" and change his calculus regarding a bloody civil war in which he has been reluctant to intervene.
If Congress votes down a resolution authorizing force, the president could risk further damage to his credibility if he doesn't follow through on his warnings to Assad. But moving forward against the will of Congress could worsen his already difficult relationship with Republicans and jeopardize the rest of his legislative agenda.
Earlier Friday, White House deputy national security adviser Tony Blinken said of the president that it is "neither his desire nor his intention to use that authority absent Congress backing him." Obama deflected a question about the remark during his news conference, again refusing to give a yes-or-no answer about what he would do if Congress turns him down.
On the ground in Syria Friday, a monitoring group said the government sent reinforcements, including tanks and armored personnel carriers, to a predominantly Christian village north of Damascus where rebels have battled government troops this week. Opposition fighters led by an al-Qaida-linked rebel faction had attacked the ancient mountainside sanctuary of Maaloula and briefly entered the village. The assault has spotlighted fears among Syria's religious minorities about the prominent role of Islamic extremists in the rebel ranks fighting to overthrow Assad.
Meanwhile, the Kremlin said Russia was boosting its naval presence in the Mediterranean Sea, moving warships into the area. That was stoking fears about a larger international conflict if the United States orders airstrikes.
The U.S. already has five Navy destroyers armed with Tomahawk missiles on standby in the Mediterranean.
Associated Press writers Nataliya Vasilyeva, Angela Charlton and Josh Lederman in St. Petersburg, and Donna Cassata and Nedra Pickler in Washington contributed to this report.
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