As the Obama administration's national security team prepared to make its case to Congress for a strike on Syria, and President Obama spent his morning doing the same in private meetings, it appears the key phrase being employed as their rationale for a missile attack is "it's in our national security interest."

Top congressional leaders echoed this claim as they, one by one, got behind the administration's call for a military strike on Tuesday morning.

A senior State Department official told Fox News that Secretary of State John Kerry, who is testifying before a Senate panel Tuesday afternoon, will also "be making the case about why it's in our national security interest to take action against Syrian regime targets, to deter and degrade the regime's ability to use chemical weapons in the future and hold it responsible for this heinous act."

House Speaker John Boehner, emerging from a White House meeting with several other top-ranking lawmakers, said the chemical weapons attack last month "has to be responded to."

"The use of chemical weapons is a barbarous act. It's pretty clear to me that the United Nations is unable to take action, NATO not likely to take action. The United States for our entire history has stood up for democracy and freedom for people around the world," Boehner said. "I'm going to support the president's call for action."

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Minutes later, House Democratic Leader Nancy Pelosi told reporters that she, too, feels Syrian President Bashar Assad has "crossed a line."

"President Obama did not draw the red line. Humanity drew it," Pelosi said. She said lawmakers will have to decide, but she thinks "we must respond." She said she does not think Congress will reject the president's request for authorization. She later told her caucus that it's "in the national interest" to respond on Syria.

House Republican Leader Eric Cantor said definitively that he plans to vote in favor of giving Obama authorization to use military force.

"While the authorizing language will likely change, the underlying reality will not. America has a compelling national security interest to prevent and respond to the use of weapons of mass destruction, especially by a terrorist state such as Syria, and to prevent further instability in a region of vital interest to the United States," he said in a written statement.

The statements backed Obama's argument that a limited missile strike on the Assad regime would be in the U.S. national security interest, by deterring the use of chemical weapons. Obama, as the White House meeting began, said he was confident he would win congressional support.

Others, though, have cast the president's decision to seek congressional backing as a big risk. Some have described his chances of winning approval as "50-50." While congressional leaders and senior members of key committees are getting onboard, the president faces skepticism from both liberal Democrats and some Tea Party-aligned Republicans. Others worry the administration is not going far enough with a limited strike that the administration admits is not aimed at regime change.

Senate Democratic Leader Harry Reid did not attend the Tuesday morning meeting at the White House, but a Reid aide told Fox News that he, too, is strongly in favor of taking action against the Syrian government. Senate Republican Leader Mitch McConnell has not said whether he supports military action.

The endorsements come ahead of a planned hearing Tuesday afternoon before the Senate Foreign Relations Committee. Kerry, Secretary of Defense Chuck Hagel and Chairman of the Joint Chiefs of Staff Gen. Martin Dempsey are set to testify on the rationale for military action in Syria.