President Barack Obama will take his case for a military attack on Syria directly to the American people in a nationwide address Tuesday, he told reporters in Russia at the conclusion of the G20 summit.
Facing public opposition reflected by a Congress hesitant to support him, Obama said Friday that he understands the skepticism over what he labeled "limited" and "proportional" military strikes intended to punish the regime of Syrian President Bashar al-Assad for what U.S. officials call a major chemical weapons attack on August 21 that killed more than 1,400 people.
"The American people have gone through a lot when it comes to the military over the last decade or so," Obama said.
He also cited a responsibility borne by the United States as a global power to lead what he hopes would be an international response in order to maintain the credibility of treaties and conventions against weapons of mass destruction.
"I believe when you have a limited proportional strike like this, with manageable risks, then we should bear that responsibility," Obama said.
The president also said that "we will be more effective if we are unified moving forward" in explaining why he sought congressional approval for what he argues is a necessary response to the violation of international norms by al-Assad's regime.
Opposition by permanent Security Council members Russia and China has scuttled Obama's hopes for U.N. authorization of a military response against al-Assad's regime.
His speech to the nation Tuesday will follow an aggressive outreach strategy to woo members of Congress to back his pitch for limited strikes, expected to be missile attacks at military command targets but not chemical weapons stockpiles.
After two congressional hearings this week, a growing number of lawmakers have said they'll vote against giving the president authorization for military action, and a majority of the Senate and House remain "undecided," according to CNN's latest count.
"I knew this was going to be a heavy lift," Obama said during Friday's news conference in St. Petersburg, Russia.
The Obama administration has had discussions with at least 60 senators and 125 House members in the past two weeks as the president seeks congressional approval for his pitch to strike Syria, a White House official confirmed Friday.
On Thursday alone, while the president was in Russia for the G20 Summit, he and senior administration officials made more than 25 individual calls to what the White House described as bipartisan members of the House and Senate. Obama called five senators himself, the White House said. Officials refused to reveal the names of the recipients.
Should Congress reject his request to authorize military strikes, Deputy National Security Adviser Tony Blinken said Friday on NPR that "the president of course has the authority" to act in Syria without support from Capitol Hill, but "it's neither his desire nor intention to use that authority absent Congress backing him."
A senior administration official from the National Security Council later clarified his remarks, saying "the president's intention is to act with congressional authorization, and we believe they will vote to provide that authorization."
Asked several times at the news conference about Blinken's remark and whether he would attack Syria anyway without authorization, Obama acknowledged that he would avoid providing a direct answer.
"It would be a mistake for me to jump the gun and speculate because right now I'm working to get as much support as possible out of Congers," he said.
On Wednesday, the Senate Foreign Relations Committee approved an amended version of the White House's resolution for authorization to attack Syria by a 10-7 vote. The full Senate will debate the resolution next week, and the House also will be in session to consider the issue.
Approval is considered more likely by the Democratic-led Senate than the Republican-led House, but the high level of public opposition makes the outcome unclear in either chamber.
CNN's Kevin Bohn and Gloria Borger contributed to this report.