President Obama will make his case for a strike on Syria with an address to the nation on Tuesday, as opposition grows to U.S. military involvement in that country’s two-year civil war.

Obama announced his planned speech during a press conference at the G-20 summit in St. Petersburg, Russia, acknowledging that his pitch was a “hard sell” but “something I believe in.”

“Making sure the American people understand it is important before I take action," Obama said of his decision to address the country.

The president has sought congressional approval for a military strike and has worked to rally international support at the conference. But despite intense lobbying from Vice President Biden, Secretary of State John Kerry, Defense Secretary Chuck Hagel — and even Obama himself — the White House is struggling to win support in Congress.

Congressional leaders, including House Speaker John Boehner, R-Ohio, and Minority Leader Nancy Pelosi, D-Calif., have backed the president, but many rank-and-file members in both parties are reluctant to support the president, in the face of polls showing little enthusiasm for military engagement among the public.

The administration has intensified its push, with Obama personally calling lawmakers from Europe and canceling a trip to California scheduled for next week to remain in Washington and focus on Syria.

The president acknowledged challenges on Capitol Hill, but refused to speculate on what actions he would take if lawmakers rebuffed his call for a strike on Syria.

Earlier Friday, Deputy National Security Adviser Tony Blinken suggested Obama would not act alone on Syria if Congress rejected his appeal. But the White House said only that they were confident lawmakers would back the president and approve a strike.

Some Republicans who back the president on Syria though are questioning whether it is now too late to sway the public and lawmakers on the contentious issue.

“Members of Congress represent the views of their constituents, and only a president can convince the public that military action is required,” Brendan Buck, a spokesman for Speaker Boehner said Thursday. “We only hope this isn’t coming too late to make the difference.”

Obama said he understood the apprehension on Capitol Hill, noting the American public was wary of yet another military entanglement abroad. But the president insisted that any military response would be “limited and proportional,” attempting to draw a distinction between an operation against Syrian President Bashar Assad and the U.S. wars in Iraq and Afghanistan.

Obama answered questions from reporters after a surprise private meeting with Russian President Vladimir Putin on the sidelines of the G-20 summit.

The two leaders have a frosty relationship marked by sharp disagreements over Syria as well as Russia’s refusal to turn over National Security Agency leaker Edward Snowden. Putin and Obama failed to reach a consensus on how to respond to the violence in Syria.

“We stuck to our guns," Putin told reporters, warning that Russia would continue to supply Syria, a key ally of Moscow, with aid and weapons in the event of a U.S. strike.

Obama had hoped to mobilize strong international support for an attack on Syria at the G-20 summit to bolster his sales pitch at home. The White House says it has the support of G-20 leaders from Australia, Canada, France, Italy, Japan, South Korea, Saudi Arabia, Spain, Turkey and the United Kingdom.

Those nations signed a joint statement condemning a chemical weapons attack by Assad’s forces and urging an international response.

However, nearly half of the nations at the international gathering did not sign on to the statement, and one signatory, the United Kingdom will likely not join in any military action after Parliament last week rejected Prime Minister David Cameron’s call for a strike.

Obama said the United States and the international community could not afford to sit by as Assad deploys chemical weapons against his own people.

"Failing to respond to this breach of this international norm will send a signal to rogue nations that they can develop and use weapons of mass destruction and not face consequences,” said Obama.