The US secretaries of state and defense go before a Senate panel seeking support to attack Syria over its alleged use of chemical weapons, warning failure to do so might embolden Hezbollah and Iran.
In what will be one of the most high-profile political set pieces in Washington in weeks, John Kerry and Chuck Hagel will testify to the Senate Foreign Relations committee, on Tuesday.
America's top military officer, General Martin Dempsey, chairman of the Joint Chiefs of Staff, will also go before the panel.
Kerry will argue that failing to act in Syria "unravels the deterrent impact of the international norm against chemical weapons use," a senior State Department official said on condition of anonymity.
Inaction also "endangers our friends and our partners along Syria's borders... and risks emboldening Assad and his key allies - Hezbollah and Iran," the official warned.
As the White House battles for congressional authorization to bomb Syria, two top Republicans warned Monday that a "no" vote after President Barack Obama had threatened action would be catastrophic.
Hawkish senators John McCain and Lindsey Graham emerged from an hour of talks with Obama suggesting that the White House could be mulling a wider military campaign in Syria than first thought, along with more robust support for the opposition.
Obama shocked Washington and the world on Saturday when he decided to seek support for military action in Syria from Congress, when it seemed US cruise missile strikes on President Bashar al-Assad's forces and assets were imminent.
McCain and Graham appeared to offer qualified backing for Obama's plans.
"A vote against that resolution by Congress I think would be catastrophic," said McCain.
"It would undermine the credibility of the United States of America and the president of the United States. None of us want that."
Graham warned of the wider consequences of a failure to back military action.
"I can't sell another Iraq or Afghanistan, because I don't want to," Graham said.
"(What) I can sell ... (is) that if we don't get Syria right, Iran is surely going to take the signals that we don't care about the nuclear program."
The senators also hinted at the administration's evolving strategy for Syria.
Obama has stressed that any US action, expected to include cruise missile attacks, would be "limited" and "narrow."
But McCain said he had "been given some reason to believe that very serious strikes may take place as opposed to cosmetic (ones)."
"I don't think it is an accident that the aircraft carrier is being moved over in the region," he said.
ABC News reported Monday that the USS Nimitz aircraft carrier was moving westward toward the Red Sea, though had not yet received orders to support a strike on Syria.
Obama's plan could be to degrade Assad's capabilities and upgrade those of vetted opposition groups, McCain suggested.
Graham indicated that the administration, which resisted arming Syrian rebels for months, may be prepared to stiffen a nascent plan -- announced after previous, small-scale chemical weapons attacks -- to increase military aid to some rebel groups.
"There seems to be emerging from this administration a pretty solid plan to upgrade the opposition," Graham said, saying regional players like Saudi Arabia, Turkey and Jordan should play a key role.
The White House declined to comment on the suggestions of a wider operation or increased aid to the rebels, but it also did not challenge them.
Though McCain and Graham's comments would have pleased the White House, they have long supported military action against Syria so are an easier sell than many other Republicans.
The administration may have its work cut out for it in the Republican-run House of Representatives, which includes many conservatives who have blocked Obama's agenda at home on issues like gun control and immigration reform and may relish the chance to embarrass him abroad.
Liberal House Democrats wary of another prolonged engagement in the Middle East are also a cause for White House concern.
With that in mind, top administration officials including Kerry and Hagel held an unclassified briefing for House Democrats on Monday, while most of official Washington was on hiatus for the Labor Day holiday.
A senior White House official said Kerry will also testify on Wednesday before the House Foreign Affairs committee. Classified briefings will meanwhile be offered to lawmakers throughout the week.
Kerry has also continued his flurry of phone calls to US allies in the region, including Israeli Prime Minister Benjamin Netanyahu, with whom he spoke on Sunday, the State Department said.
Obama has a limited window this week for personal politicking on Syria: he is due to leave town on Tuesday evening to visit Sweden and attend the G20 summit in Russia.
A senior White House official said the administration would deploy all of its resources on Capitol Hill and beyond to sway opinion on the Syria vote, and the president plans to meet with committee leaders and opposition heads from both the House and Senate before departing for Europe.