In a bid to rally support for attacking Syria, President Obama plans to make a prime-time address from the White House on Tuesday to present his case directly to the American people.

The speech comes as Obama faces mounting opposition to launching a military strike — especially in the Republican-controlled House — raising the threat that Congress could reject Obama's request for authorization. A Reuters/Ipsos poll this week found that 56 percent of the American public also doesn't think the U.S. should intervene.

With such tepid backing, Obama's allies in Congress have been urging him to formally lay out his reasoning. Indeed, some have warned that opposition has solidified in the public and on Capitol Hill, and it might be too late for Obama to change many minds, says Jennifer Epstein at Politico.

Here are four things Obama is likely to say to win over skeptics:

1. This is bigger than Syria
At the G-20 summit in St. Petersburg, Russia, this week, Obama got 10 nations to sign a document to "support the efforts undertaken by the United States and other countries to reinforce the prohibition on the use of chemical weapons." He said on Friday that he wants to make Americans see that the recent sarin-gas attack outside Damascus, which the White House says killed 1,400 people, "isn't just a Syrian tragedy. It's a threat to global peace and security."

Chemical weapons kill indiscriminately, and have been banned under international treaties for generations. The recently reported use of sarin nerve agent behind Syrian rebel lines was the most deadly use of such weapons since Saddam Hussein gassed Kurds and Iranians in the 1980s.

Obama will argue that reinforcing the taboo against chemical warfare is necessary, to prevent these attacks from becoming more common. "I want people to understand that gassing innocent people, delivering chemical weapons against children, is not something we do," Obama said. He added, "The kind of world we live in and our ability to deter this kind of outrageous behavior is going to depend on the decisions that we make in the days ahead."

2. The world can't count on the United Nations
Russian officials insisted this week that a U.S. strike against Syria, without approval from the United Nations, would "drive another nail into the coffin of international law." The Obama administration's U.N. ambassador, Samantha Power, countered by saying that the Security Council was no longer a "viable path" for holding Syria accountable for war crimes, because Syrian President Bashar al-Assad's ally Russia, along with China, are using their veto power to block any action against him.

Expect Obama to reiterate that argument, which he made on Friday before leaving Russia. "It is my view," he said, "that given Security Council paralysis on this issue, if we are serious about upholding a ban on chemical-weapons use, then an international response is required and that will not come through Security Council action."

3. This time, the U.S. really has proof
Obama is facing heightened public cynicism thanks to the intelligence fiasco that preceded the Iraq War. "So here is a modest proposal," says Jeffrey Lewis at Foreign Policy. "Why not just release the vast majority of the evidence? We have a transcript of some Syrian army officer being ordered to gas Ghouta? Release it. (Newsflash: The Syrians know we monitor their communications.) We have satellite images of the units in place during the attack? Release them. (Another newsflash: Other countries know we have imaging satellites with very high resolution.) Release the vast majority of data, with basic information and commentary that would allow the rest of us to make up our own minds."

Either way, the evidence that Assad used chemical weapons is supposedly quite strong, a point that Obama will have to reinforce if he wants to drag Syria out of Iraq's long shadow.

4. Sometimes military action is the right thing to do
Expect Obama to make the argument that he has spent his presidency trying to end the wars in Afghanistan and Iraq, says Matt Vasilogambros at National Journal, but that sometimes military action is necessary — even it is the last thing the public wants to do. Here is how Obama put it on Friday:

When London was getting bombed [in World War II], it was profoundly unpopular, both in Congress and around the country, to help the British. It doesn't mean it wasn't the right thing to do. It just means people are struggling with jobs and bills to pay and they don't want their sons or daughters put in entanglements far away are dangerous and different. To bring the analogy closer to home, the intervention in Kosovo, very unpopular, but, ultimately I think it was the right thing to do and the international community should be glad that it came together to do it.