President Donald Trump risks a political backlash if he retaliates against Iran over a weekend strike on Saudi Arabian oil facilities after campaigning on promises to withdraw the U.S. from foreign wars.

Trump tweeted Sunday that the U.S. is “locked and loaded,” raising bipartisan alarm after the weekend attack that halved Saudi oil production. Speaking on Monday to reporters in the Oval Office, Trump said, “We pretty much already know” who did it.

Secretary of State Mike Pompeo has blamed Iran, though Riyadh is stopping short of directly doing so.

Iran has denied involvement and it’s supreme leader, Ayatollah Ali Khamenei, said today that the Islamic Republic won’t negotiate with the U.S. on any level, including at this month’s annual United Nations General Assembly in New York.

Trump, who hasn’t ruled out a possible meeting with Iran’s President Hassan Rouhani when both are in New York this month, has said diplomacy with Iran isn’t exhausted and that Pompeo will be heading to Saudi Arabia.

The prospect of a strike on Iran in retaliation for an attack that didn’t target Americans or even a country with which the U.S. has a defense treaty is proving divisive in Washington. Trump said the U.S. is prepared for any conflict, but that Saudi Arabia would need to play a significant part in any action.

Saudi Arabia’s popularity is at a nadir over the prolonged war it has fought in Yemen that has killed thousands of civilians and the murder of journalist Jamal Khashoggi in the kingdom’s Istanbul consulate earlier this year. Congress voted in April to cut off military support for Saudi Arabia over the Yemen war, legislation Trump vetoed.

Sen. Lindsey Graham, a South Carolina Republican, said Monday that Trump should consider attacking Iranian oil refineries, but Sen. Mitt Romney, a Utah Republican, said direct U.S. intervention would be “a grave mistake.”

Before he was president, Trump expressed skepticism of U.S. military action on behalf of Saudi Arabia. “Saudi Arabia should fight their own wars, which they won’t, or pay us an absolute fortune to protect them and their great wealth-$ trillion!” he said in a 2014 tweet.

But his tweet on Sunday indicated that he’s waiting for the kingdom to tell the U.S. what it wants to do.

Trump adopted the slogan “America First” during the 2016 campaign, and vowed to withdraw from foreign wars.

Defense Secretary Mark Esper said that defense officials had briefed Trump about the situation on Monday, and that Esper spoke with Saudi Crown Prince Mohammad bin Salman and Iraqi Defense Minister Najah al-Shammari over the weekend.

“The United States military, with our interagency team, is working with our partners to address this unprecedented attack and defend the international rules-based order that is being undermined by Iran,” Esper tweeted.

Marc Short, the chief of staff to Vice President Mike Pence, earlier downplayed the prospect of U.S. military action.

“I think ‘locked and loaded’ means several things. One thing that it means is that America today under this president is far better prepared to handle these sort of events because we’re now a net exporter of oil,” Short said in an interview on Fox Business Network. “We know that Iran has been trying to instigate attacks against Saudi Arabia for some time.”

Sen. Chris Coons, a Democrat and member of the Senate Foreign Relations Committee, said that military action against Iran may be warranted, depending on information gathered by U.S. intelligence services.

“My hope is that the president will consult with his generals, his diplomats, his advisers, will look hard at the intelligence,” Coons said in a Monday interview on “Fox and Friends,” a show the president often watches. “Iran is one of the most dangerous state sponsors of terrorism. This may well be the thing that calls for military action against Iran if that’s what the intelligence supports.”

But Coons faced criticism from liberals for his remarks, and a Democratic colleague, Hawaii Sen. Brian Schatz, said on Twitter: “There is no imminent threat and the U.S. military is not authorized to retaliate on behalf of another country.”

Most of the vetoes Trump has issued during his presidency were of bipartisan resolutions regarding Saudi Arabia, and every time Congress has failed to muster enough votes to override his veto.

One of the measures sought to end U.S. assistance for Saudi Arabia’s war against Houthi rebels in Yemen, citing the humanitarian disaster caused by the conflict. Trump also vetoed three resolutions to curb U.S. arms sales to Saudi Arabia.

The rift between Trump and some Republicans began after the murder of Khashoggi, a U.S.-based columnist for the Washington Post. The Senate is still considering punishment for Saudi officials alleged to be involved in the killing, including a bipartisan measure to suspend arms sales to the kingdom and impose sanctions on those found responsible for the Khashoggi murder, potentially including Prince Mohammed.

James Carafano, a foreign policy analyst at the Heritage Institute, a conservative research group, said Iran needs to be held accountable for its behavior, but that the U.S. would be better off supporting a Saudi Arabian response, rather than launching its own.

“The Saudis need to have some backbone. This was not an attack on the U.S. forces or U.S. bases; this was an attack on Saudi Arabia,” Carafano said. “If you’re going to conduct a retaliatory strike, you have to make the case to the world that this is warranted and appropriate.”