Sen. Lindsey Graham Lindsey Olin GrahamLincoln Project mocks Lindsey Graham's fundraising lag with Sarah McLachlan-themed video The Hill's Morning Report - Sponsored by Facebook - Republicans lawmakers rebuke Trump on election Trump dumbfounds GOP with latest unforced error MORE (R-S.C.) on Wednesday warned that President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE's plan to increase sanctions against Iran likely won't be enough to change the country's behavior.

Graham, speaking with reporters for the first time since Trump called him out amid a scuffle on Iran, predicted that new sanctions against Iran "will fall short."

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"The maximum pressure campaign has worked in the sense it’s crippled the regime’s economy, it’s made life difficult for the regime, but it has not changed their behavior," Graham told reporters.

"The only conclusion I think you can reach is that the Iranians, while having been hurt by the maximum pressure campaign, have not been deterred in terms of their provocative behavior. And it’s going to take something, I think, beyond sanctions to achieve that deterrence," he added.

Pressed if he was saying he he believed Trump's plan to ramp up sanctions would not be enough, he added: "In the past they haven't been, but time will tell. ... My belief is that additional sanctions will fall short."

Trump announced on Twitter that he had instructed Treasury Secretary Steven Mnuchin Steven Terner MnuchinThe Hill's Morning Report - Sponsored by Facebook - Republicans lawmakers rebuke Trump on election On The Money: Anxious Democrats push for vote on COVID-19 aid | Pelosi, Mnuchin ready to restart talks | Weekly jobless claims increase | Senate treads close to shutdown deadline Vulnerable Democrats tell Pelosi COVID-19 compromise 'essential' MORE to “substantially” increase economic sanctions on Iran. Administration officials have blamed Iran for drone attacks on two Saudi oil sites over the weekend.

“I have just instructed the Secretary of the Treasury to substantially increase Sanctions on the country of Iran!” Trump tweeted.

Graham's skepticism comes after Trump lashed out at him in a tweet on Tuesday night after the South Carolina Republican criticized his policy on Iran.

Graham and Trump have fundamentally different foreign policy views. Graham is viewed as a hawk, while Trump ran in 2016 on an isolationist foreign policy. But the back-and-forth marks a rare split in their relationship after Graham went from being a vocal Trump critic in 2016 to one of his closest allies on Capitol Hill.

Graham on Tuesday had criticized Trump’s past response to a drone incident for being insufficient. Trump had authorized a military strike on Iran in response to the downing of the drone but said he called it back after learning there would be 150 casualties.

“The measured response by President Trump regarding the shooting down of an American drone was clearly seen by the Iranian regime as a sign of weakness,” Graham said, adding it was “imperative” that the U.S. take decisive action to deter further Iranian aggression.

Trump fired back on Twitter, calling out Graham by name over his comments.

"No Lindsey, it was a sign of strength that some people just don’t understand!" Trump tweeted.