Sen. Lindsey Graham, R-S.C., told reporters Sunday that "I feel pretty good about where we're headed" in Syria after suggesting that President Trump is "reconsidering" the planned pullout which had drawn bipartisan criticism and forced the resignation of Defense Secretary James Mattis.

Graham emerged from the White House after a two-hour lunch with Trump and said the president "told me some things I didn't know that make me feel a lot better about where we're headed in Syria."

"He promised to destroy ISIS. He's going to keep that promise," Graham said of Trump. "We're not there yet. But as I said today, we're inside the 10-yard line and the president understands the need to finish the job."

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Earlier Sunday, Graham called on Trump to reverse his decision to withdraw U.S. troops from Syria in a wide-ranging interview broadcast on CNN's "State of the Union."

"If we leave now, the Kurds are going to get slaughtered," Graham told host Dana Bash, adding that Trump had discussed the matter with Chairman of the Joint Chiefs of Staff Gen. Joseph Dunford and was "reconsidering how we do this."

"He's frustrated. I get it. People should pay more. They should fight more," Graham said. "But we're not the policemen of the world here. We're fighting a war against ISIS. They're still not defeated in Syria. I'm asking the president to make sure that we have troops there to protect us. Don't outsource our national security to some foreign power. If we leave now, the Kurds are going to get into a fight with Turkey, they could get slaughtered."

Graham echoed that theme outside the White House, telling reporters that the Kurds "stepped up when nobody else would" to fight ISIS.

"The last thing in the world we want is a war for Turkey and the Kurds," Graham said. "That takes pressure off ISIS. The last thing we want in addition to that is Iran to be the big winner here. So I think the president's going to finish the job when it comes to ISIS. I share his goal to withdraw our forces from Syria. I just want to do it in a smart way to make sure that Iran’s not the big winner."

In his interview with CNN, Graham also characterized former President Barack Obama's justification for pulling troops out of Iraq in 2011 -- a move widely credited with allowing the rapid rise of the terrorist group ISIS there -- as a "bunch of bullsh--, pardon my French."

Graham rejected the idea that the 2008 U.S.–Iraq Status of Forces Agreement, signed by President George W. Bush, had forced Obama's hand. The agreement called for all U.S. forces to leave the country by the end of 2011.

“That’s a complete and absolute lie," Graham said. "I was there, talking to the prime minister of Iraq -- Obama wanted to get to zero. He got to zero. October 21, 2011, I said I hope the president is right, but I fear this decision will come back to haunt us. ISIS came about as a result of our withdrawal from Iraq."

Everything we're dealing with today falls on Obama's watch," Graham added. "He's the one that withdrew from Iraq."

After lunch with the president, Graham said that he thought Trump's post-Christmas visit to Iraq was "eye-opening."

"The commanders there told him that ISIS was in a world of hurt," the senator said. "Not completely destroyed but well on their way. I think operations to completely destroy and decimate ISIS are going to be ongoing and are going to be accelerated.

"So the president assured me that he's going to make sure he gets the job done and I assured him that nobody has done more to defeat ISIS than he has," Graham added.

Graham later tweeted: "I learned a lot from President @realDonaldTrump about our efforts in Syria that was reassuring. The President will make sure any withdrawal from Syria will be done in a fashion to ensure: 1) ISIS is permanently destroyed. 2) Iran doesn’t fill in the back end, and 3) our Kurdish allies are protected.

"President @realDonaldTrump is talking with our commanders and working with our allies to make sure these three objectives are met as we implement the withdrawal," Graham concluded.

In a video shot outside the White House and posted to his Twitter account earlier this month, Trump explained his sudden decision to pull troops out of Syria hours earlier in unusually personal terms, saying he has found it increasingly difficult to inform soldiers' loved ones that their kin had died in combat.

"We've been fighting for a long time in Syria," Trump said. "I've been president for almost two years, and we've really stepped it up. And we've won against ISIS. We've been them, and we've beaten them badly. And now it's time for our troops to come back home."

Mattis, who signed orders to pull all American troops out of Syria in the coming weeks, gave his resignation letter to President Trump earlier this month, in which he acknowledged that a strong nation relies on a "comprehensive" network of alliances, and must be "resolute and unambiguous" in approaching countries with strategic differences, citing China and Russia.

"Because you have the right to have a Secretary of Defense whose views are better aligned with yours on these and other subjects, I believe it is right for me to step down from my position," Mattis wrote.

A senior U.S. defense official told Fox News that Mattis was leaving "in protest over the president's national security policies.”

Fox News' Jennifer Griffin and Lucas Tomlinson contributed to this report.