Donald Trump, Sen. Ted Cruz (R-Tx), Sarah Palin and other political leaders gathered before a crowd in front of the Capitol building in a rally against the Iran nuclear deal. (Alice Li/The Washington Post)

The anti-establishment disenchantment now roiling the Republican presidential nominating contest landed on Congress’s doorstep Wednesday, with two presidential candidates rallying opposition to the Iran nuclear deal on the Capitol lawn, targeting not just President Obama but GOP leaders as well.

“We are led by very, very stupid people — very, very stupid people,” front-runner Donald Trump told the crowd, eliciting hearty cheers. “We cannot let it continue.”

The crowd frequently yelled “Amen!” and consistently booed any mention of Obama, congressional Democrats, House Speaker John A. Boehner ­(R-Ohio) and Senate Majority Leader Mitch McConnell (R-Ky.).

Meanwhile, inside the Capitol, a member revolt forced Boehner to revise his plans to hold a vote against the Iran deal this week — an unexpected showing of the internal GOP acrimony that has some fearing a government shutdown in three weeks.

Republicans have been united against the Iran deal, and a disapproval resolution was expected to pass the House easily. But dissension emerged over tactics, turning the Iran agreement into the latest high-stakes issue to expose sharp divides inside the GOP.

In the Senate, Democrats appeared resolved to maintain the 41-vote firewall necessary to keep opposition legislation from passing the chamber and advancing to Obama’s desk. They also gawked at the GOP infighting with a mix of horror and fascination.

“Everywhere Republican leaders look this fall, there’s potential disaster lurking thanks to their hard-right members determined to hold the government hostage unless they get everything they want,” said Sen. Charles E. Schumer (D-N.Y.).

The determination among the most conservative GOP members to confront their leadership appears to have been stoked by the fervor that has greeted politicians such as Trump and Sen. Ted Cruz (R-Tex.), the presidential candidates who headlined the Capitol Hill rally Wednesday. Both have been equally comfortable railing against Obama and the established Republican leadership, which was dealing with emboldened conservatives on Wednesday.

After a morning caucus meeting, Rep. John Fleming (R-La.), a member of the hard-line Freedom Caucus, said he and colleagues were considering opposing a routine procedural vote to bring the Iran disapproval to the House floor, blocking its consideration.

Shortly before noon, House Republican leaders canceled the scheduled procedural vote as a concession to conservatives and called a late-afternoon caucus meeting to discuss a path forward. Members emerged agreeing on a new plan to vote on a trio of measures designed to register disapproval with the president: a resolution indicating that Obama did not meet his obligations to send all relevant negotiating documents to Congress; a bill blocking Obama from lifting sanctions against Iran; and a separate measure approving of the deal, which is expected to fail.

With Democrats poised to block any Senate action, the change in tactics is unlikely to have any practical effect on the implementation of the nuclear deal. But several House members leaving the closed-door session said the process will allow them to vote against Obama’s foreign policy in Iran and establish grounds to later sue the White House on grounds that it illegally negotiated the deal.

There is growing worry that the unrelenting contentiousness will be the hallmark of yet another fall legislative season, just as funding authority for federal government agencies is set to expire. If conservatives balk at supporting legislation to extend agency funding past Sept. 30, the federal government could be headed for its second shutdown in three years.

Duck Dynasty star Phil Robertson and radio personality Glenn Beck joined a long list of politicians at the Capitol on Wednesday to express their disapproval with the international nuclear deal with Iran. (AP)

Out in front of the Capitol, Cruz, who invited Trump to the rally, embraced the real estate mogul onstage and praised him as “my friend.”

“Donald has an incredible ability to attract attention,” Cruz told reporters after the rally.

Hundreds of people stood in sweltering heat on the West Lawn of the Capitol. Many carried signs bearing phrases such as “#JewishLivesMatter Ask God” and “Give them hell Ted Cruz.” People waved huge American and Israeli flags. A group of bagpipers played patriotic songs. And a woman held a punching bag with a cartoon of Obama on it; a man walked up and asked if he could hit it.

“Can I punch him, please?” asked Jim Scearce of Danville, Va. “My only dream is that could have been real life.”

The woman holding the punching bag, Rose Prescott of Phoenix, was on vacation in Virginia and made sure to attend the rally in order to “tell Congress to grow a pair and do what the people want.”

“We don’t trust Iran,” she said. “They are liars. Their goal is to take out Israel.”

The deal reached last month between Iran and the United States and five other countries would require Iran to abandon its quest for nuclear weapons over the next 15 years in exchange for relief from financial sanctions.

Cruz, who organized the event alongside the Tea Party Patriots and the Zionist Organization of America, mainly took aim at Obama and Senate Democrats: “How will you look in the eyes of the mother or father or sons or daughters of those who are murdered by jihadists, those Americans who are blown up, those Americans who are shot, those Israelis who are murdered? . . . You cannot wash your hands of that blood.”

He also had sharp words for his party’s leaders: “There are two men in Washington, D.C., who could defeat this deal. . . . Mitch McConnell and John Boehner can stop this deal if they simply enforce federal law.”

That statement came in solidarity with a bloc of House conservatives who seized on a floor resolution filed Tuesday by Rep. Peter J. Roskam (R-Ill.) contending that Obama did not comply with a review law passed this year requiring the submission to Congress of not only the text of the Iran deal but also “all related materials and annexes.”

Because “side deals” between Iran and the International Atomic Energy Agency establishing the parameters for weapons inspections and investigations of past military dimensions of the Iran nuclear program have not been released, Roskam and his allies say, Congress has no business voting up or down on the deal.

That argument was embraced by the hard-line lawmakers who have been increasingly at odds with Boehner and who demanded that the House assert that Obama has defied the review law.

“The endgame is that the president either puts up or shuts up,” Fleming said. “We might find out some things in that [IAEA side agreement] that are absurd, and even Democrats wouldn’t be willing to vote for.”

But Boehner loyalists said Wednesday that they saw little point in delaying the planned Friday vote. Senate leaders also showed no sign that they would stop pressing a disapproval bill.

“It will be perceived by the American people that we abdicated,” said Rep. Peter T. King (R-N.Y.). “I think we have an obligation to be heard on this, to vote on it. . . . Otherwise the average person is just going to think we’re afraid.”

King said he doubted the side deals would make any difference in the outcome of the Iran debate: “I understand the point they’re trying to make, but it’s not going to change anyone’s vote.”

While Republican lawmakers were waging a battle with each other inside the Capitol, the GOP’s unlikely standard-bearer of the moment was outside winning cheers and applause.

Trump took the stage to R.E.M.’s “It’s the End of the World as We Know It,” then he proceeded to lament how America “can’t beat anybody” and vowed to change that if elected president.

“We will have so much winning if I get elected,” he said, “that you may get bored with winning.”

Karoun Demirjian and Kelsey Snell contributed to this report.

Correction: A previous version of this article gave John Boehner’s title as House majority leader. He is speaker of the House.