Sen. Lindsey Graham (R-SC) told The Daily Beast on Tuesday that the Trump administration’s campaign of “maximum pressure” on Iran isn’t achieving its purpose.

“Clearly what we’re doing isn’t working,” Graham said, adding that the goal of the administration’s Iran policy should be to deter Tehran from escalating tensions in the region.

Following his phone call with The Daily Beast, Graham, in a series of tweets, called on the Trump administration to do more, saying the “measured response” by President Trump is “seen by the Iranian regime as an act of weakness.” On the same night as a Tuesday fundraising event in Silicon Valley, Trump tweeted back at Graham: “No Lindsey, it was a sign of strength that some people just don’t understand!”

The Trump administration has carried out its “maximum pressure” campaign against Iran since its earliest days, enlisting the Treasury and State departments to annihilate the country’s economy via sanctions. With each week, the administration has seemed to find a new Iranian company or sector to punish financially.

That strategy has been put to the test in recent months. Earlier this summer, the Trump administration accused Tehran of sabotaging tankers in the Strait of Hormuz. Since then, Democrats on Capitol Hill have called on Team Trump to change course, saying the campaign to deter Iran via sanctions was falling flat.

One of the main architects of the White House’s strategy was former National Security Adviser John Bolton, the openly hawkish politico who has supported the idea of American military action against the Iranian regime for years. With his abrupt departure last week, the Trump team has leaned increasingly on the Pentagon for guidance on Iran. Graham seems to be filling the Bolton void, calling on the president to take “decisive action” against Tehran.

“The problems with Iran only get worse over time so it is imperative we take decisive action to deter further aggression by the Ayatollah and his henchman,” Graham tweeted Tuesday.

The divergence between Trump and Graham is notable given that the Trump administration recently enlisted the senator to engage with outside parties about what a new Iranian nuclear deal might look like.

In a previous interview with The Daily Beast, the senator said he spoke with the Trump team about the idea of asking the Iranian regime to agree to a 123 Agreement—a key, legally binding commitment that requires countries doing nuclear deals with the U.S. to sign on to nonproliferation standards. The U.S. has entered into those pacts with more than 40 countries.

Sen. Rand Paul (R-KY) has also been involved in conversations with the Trump administration on Iran strategy. Following Trump’s tweet Tuesday night, Paul’s chief strategist tweeted at Trump, saying of Graham, “DRAG HIM.”

Graham told The Daily Beast on Tuesday that he had not spoken with Trump about the attacks on Saudi oil infrastructure, but said the Israelis were “increasingly concerned” about the situation.

In an interview in August about Iran, Graham told The Daily Beast: “This is a provocative regime that shoots its own people down in the street, that threatens the existence of the state of Israel. This is an enemy of mankind, and if you are not willing to take this enemy on, you will regret it.”

It wouldn’t be the first time the president found himself disagreeing on foreign policy with the South Carolina senator, a former foe who has become a top confidant and sounding board for Trump.

In the days since the attack, Trump has indicated to those close to him that he favors restraint over military strikes, and certainly over starting a war with Iran. According to two people who’ve heard the president say this, Trump has asked advisers and senior officials why the Saudis can’t just “handle it” themselves, and has made clear he’d prefer that the United States not intervene militarily.

That could, of course, change very quickly, if recent history is any guide. In June, the president approved strikes on Iran—strikes that were projected to kill at least dozens of Iranians—only to call the whole thing off not long before the attack would have launched.

One factor driving Trump’s reluctance to bomb or go to war with Iran lies in his desire to win re-election next year.

In multiple instances during his presidency, when senior administration officials have briefed him on Iran, Trump has gone out of his way to express his worries about the possible effect that growing tensions, or a shooting war, with the Islamic Republic could have on oil markets, according to two sources familiar with his private comments. In discussing this with officials, the president has suggested that a major clash between the two countries could create a ripple effect that erases economic gains that Americans have enjoyed in recent years—gains for which Trump has taken much credit and made a cornerstone of his 2020 campaign.