Experts warned of the possibility of war as Senate Democrats prepared to introduce legislation aimed at stopping the Trump administration from attacking Iran on Wednesday, following a belligerent speech by National Security Adviser John Bolton at a meeting of anti-Iran groups.

Bolton, who helped lead the Bush administration into the war in Iraq and has pushed for the U.S. to overthrow Iran's government for years, told his audience that Iran would have "hell to pay" if the country did not change its "behavior."

President Trump's regime change industry grinds on even as he is at the UN making platitudes about peace. 'US-Iran: John Bolton warns Iran of 'hell to pay' if crossed' #UNSC https://t.co/9QB6GjGRrh — Win Without War (@WinWithoutWar) September 26, 2018

"The United States is not naïve," Bolton said. "We will not be duped, cheated, or intimidated. The days of impunity for Tehran and its enablers are over. The murderous regime and its supporters will face significant consequences if they do not change their behavior. Let my message today be clear: We are watching, and we will come after you."

Sen. Tom Udall (D-N.M.) said Tuesday afternoon that he would introduce a bill to require President Donald Trump to acquire Congressional approval for any action against Iran.

This inflammatory and belligerent speech from Trump National Security Advisor Bolton is yet another reason why I’m introducing a bill tomorrow to make sure @POTUS doesn't go to war with Iran without Congressional approval. https://t.co/inEvGPrZde — Tom Udall (@SenatorTomUdall) September 25, 2018

Bolton's speech to @UANI boiled down to its last line, directed towards Iran's leadership: "We are watching and we will come after you." Amusing how the administration denies pushing regime change while its Strangelovian advisors spew cartoon villain rhetoric like this. pic.twitter.com/H9UJHGpmgx — Max Blumenthal (@MaxBlumenthal) September 25, 2018

The speech came as tensions between the U.S. and Iran have risen over President Donald Trump's decision to exit the 2015 Iran nuclear deal, painstakingly brokered by the Obama administration and called the "best way" to ensure that Iran would not proliferate nuclear weapons by several world leaders. The United States' withdrawal from the pact was vehemently opposed by the U.N. and international leaders as well as the majority of Americans.

NSA Bolton calls Iran nuclear deal worst foreign policy decision in US history. Not decision by W Bush administration - of which he was senior member - to invade Iraq, a decision that claimed hundreds of thousands of lives, cost trillions and still is shaking the Mideast. — Jonathan Landay (@JonathanLanday) September 25, 2018

Iranian President Hassan Rouhani also blamed U.S.-backed Persian Gulf state actors for a terrorist attack at a parade that killed nearly 30 people over the weekend. Rouhani's remarks echoed a theory that was supported by analysts including Trita Parsi, president emeritus of the National Iranian American Council, in which Saudi Arabia, the United Arab Emirates, or both had a played role in the attack.

Parsi was among the critics who called Bolton's bellicose address on Tuesday the Trump administration's "way of starting war."

John Bolton on Iran today: "There will be hell to pay!" "We are watching, and we will come after you." The Trump Administration pretends that this is their way to get diplomacy going. The rest of the world fully understands that this is their way of starting war. — Trita Parsi (@tparsi) September 25, 2018

Rouhani opened the door to speaking with Trump at the U.N. General Assembly this week following the attack, but the president brushed off the possibility of a meeting, calling Rouhani "an absolutely lovely man" but saying Tuesday that he had no plans for talks—going against the wishes of more than 50 foreign policy leaders who urged Trump to forgo his administration's aggressive tactics and engage with Iran diplomatically.