“You just have to be suspicious as to why the administration would not want our participation.” Senate Majority Leader Mitch McConnell (R-Ky.) said Tuesday. (Michael Reynolds/European Pressphoto Agency)

The reaction from Washington’s foreign policy establishment was that President Obama’s authority as commander in chief had been challenged in a new and unprecedented way.

Forty-seven Republican senators, openly seeking to undercut the president, signed a letter to Iran’s leaders threatening to undo any agreement reached with the United States regarding their country’s nuclear program.

A battle quickly erupted over who was to blame for the dysfunction and how to defuse a situation that many foreign policy experts believe has become so poisonous that it has begun to damage the United States’ standing in the world.

Even against the backdrop of a long list of fiscal and foreign policy battles between the White House and Republicans, the latest standoff seemed exceptional.

“If you are a country in the Middle East or Asia relying on Washington, this raises questions about America’s predictability,” said Richard Haass, who is president of the Council on Foreign Relations and served in the George W. Bush and George H.W. Bush administrations. “I hear this all the time. I just know it makes others around the world more uncomfortable and contributes to a more dangerous and disorderly world.”

An already heated battle between the White House and Republicans over negotiations to curtail Iran’s nuclear program grew more tense when 47 Republican senators sent a letter to Iran designed to kill any potential deal. But is it treason? (Jason Aldag/The Washington Post)

The Senate Republicans’ move underscored how badly relations have deteriorated between the two parties in the months since the midterm elections gave Republicans a majority in the House and Senate. Among the other examples this year, Republicans have sought to halt Obama’s executive action on immigration by blocking funding to the Department of Homeland Security.

Then House Speaker John A. Boehner (R-Ohio) invited the Israeli prime minister to address a joint meeting of Congress in which he denounced the Obama administration’s nuclear negotiations in a rousing speech. The White House said the invitation was a breach of protocol designed to undercut the president’s ability to conduct foreign policy.

But this week’s letter from Republican senators drew the harshest reaction from the White House and Democrats. The letter, written by Sen. Tom Cotton (R-Ark.), informed Iranian leaders that any deal they struck with the Obama administration could be undone with “the stroke of a pen” by the next president or future Congresses.

[Tom Cotton emerges as a leading national security hawk]

A White House spokesman derided the letter as “reckless” and “irresponsible.” Obama said it appeared as though the Republicans were trying to make common cause with Iran’s hard-line mullahs to quash any potential agreement.

Former secretary of state Hillary Rodham Clinton weighed in Tuesday at a crowded news conference outside the U.N. Security Council chambers. Before she defended herself against allegations that she had improperly used a private e-mail account during her tenure, Clinton attacked Republicans who had signed the letter to Iran. “Either these senators were trying to be helpful to the Iranians or harmful to the commander in chief in the midst of high-stakes international diplomacy,” she said. “Either answer does discredit to the letter’s signatories.”

Many in the GOP foreign policy establishment, meanwhile, expressed disappointment over the increasingly partisan nature of U.S. foreign policy. Former senator Richard G. Lugar (R-Ind.), who previously served as chairman of the Foreign Relations Committee, described Cotton’s letter as “an unfortunate venture” and said he would have advised the freshman senator and Army veteran not to send it.

Others suggested that the belief that politics and partisanship “stop at the water’s edge” has always been more myth than reality but questioned the wisdom of the GOP letter.

“It is never a good idea for elected leaders to give foreigners, and especially foreign enemies, a formal invitation to join our domestic arguments,” said Phil Zelikow, who was a senior adviser to former secretary of state Condoleezza Rice. “It is not the conduct one would ordinarily expect from leaders of a great power.”

The controversy over the letter underscored the difficult and sensitive nature of the nuclear negotiations, which are nearing a critical juncture. The deal being negotiated with Iran would limit its capacity to enrich uranium to the point that, if Iranian leaders decided to kick out inspectors, it would take at least a year to produce enough fuel to make a nuclear weapon.

The Iranians would also have to commit to intrusive inspections by the International Atomic Energy Agency over the course of the agreement. The United States, which is negotiating the agreement with the five permanent members of the Security Council and Germany, initially asked Iran to agree to limitations on its nuclear program lasting 15 to 20 years, but Tehran has pressed for a shorter period.

Obama has said that the Iranians must agree to the broad outline of a deal by the end of this month.

“Iran is a tough, divisive issue. Everyone I know is concerned about what the Iranians are up to,” said the Council on Foreign Relations’ Haass. “Everybody is uneasy about the outcome of the negotiations. This is just as difficult a foreign policy question as you get,”

In the wake of the GOP letter, the White House and the Republican signatories both insisted that the other side is to blame for the fractious relations. Republicans accused the White House of repeated failures to consult with Congress as the negotiations progressed. “To make a deal of this magnitude with one of the worst regimes in the world without any congressional input leads you to believe he’s going to make the deal that we all fear he’s going to make,” Senate Majority Leader Mitch McConnell (Ky.) said Tuesday. “You just have to be suspicious as to why the administration would not want our participation.”

Rep. Adam Kinzinger (R-Ill.), an Iraq war veteran, said that even lawmakers who might have reservations about the Senate letter understand that it came out of a deep unease over Obama’s liberal use of his executive authority.

“I’m not going to sit here and defend the Senate letter,” Kinzinger said. “But the letter might never had been written had the administration been more open with members of the Senate and members of the House about what was going on and what their goals are” regarding the Iran talks.

Republicans and Democrats, meanwhile, summoned historical examples to show how this week’s controversy was either part of a long tradition or unprecedented, depending on their point of view.

Republicans cited President Woodrow Wilson’s efforts to negotiate European peace settlements after World War I, which were blocked by Congress. And they pointed to then-House Speaker Nancy Pelosi’s 2007 meeting with Syrian President Bashar al-Assad, whose government was accused of supporting the anti-U.S. insurgents in Iraq. At the time, Vice President Richard B. Cheney called the move “bad behavior.”

Democrats, meanwhile, pointed to the Cuban Missile Crisis, warning that it would have been unfathomable for Republicans to try to undercut President John F. Kennedy’s negotiations with Soviet leader Nikita Khrushchev. “I cannot imagine the Congress writing a letter . . . in the midst of those discussions and saying: ‘Don’t worry about this guy Kennedy. He doesn’t speak for the country,’ ” said Sen. Angus King (I-Maine). “And yet that’s essentially what happened.”

Both sides, however, agreed that something has changed in the conduct of U.S. foreign policy over the past few months. “Today there’s at the least a lack of trust between the parties,” Lugar said.

Steven Mufson and Paul Kane contributed to this report.