By Justin Sink, Bloomberg News (TNS)

WASHINGTON — At least a few of the U.S. Republican senators feeling the backlash from signing an open letter to Iran’s leaders are expressing some second thoughts.

Amid mounting criticism from allies, home-state editorial boards and colleagues who opted not to sign the missive, Senator Ron Johnson became the latest Republican to suggest he might do things differently if given another chance.

While Johnson said he stood by the content of the letter, which warned Iran that any deal they get from President Barack Obama might not outlast his term in office, he said it probably shouldn’t have been directed to leaders of the Islamic Republic.

“I suppose the only regret is who it’s addressed to,” Johnson said at a Bloomberg breakfast in Washington. The Wisconsin Republican said it may have been a “tactical error” and that the letter could have been addressed to Obama’s administration or the American people.

Arizona Senator John McCain, a prominent Republican voice on foreign affairs and national security, has said that haste and an impending snowstorm in Washington short-circuited more measured consideration of the letter.

“It was kind of a very rapid process. Everybody was looking forward to getting out of town because of the snowstorm,” McCain told Politico in an interview. “I think we probably should have had more discussion about it, given the blowback that there is.”

The White House, which was facing pushback on the Iran nuclear negotiations from some Democrats as well as Republicans, seized on the letter to argue that Republicans were making foreign policy a partisan issue. Obama and his aides have responded with a mix of scolding and disdain.

The president told reporters at the White House on Monday that the lawmakers seemed to be “wanting to make common cause with the hardliners in Iran.” In a recorded interview with the website Vice, an excerpt of which was released Friday, Obama said, “I’m embarrassed for them.”

European allies who also are party to the Iran negotiations have condemned the letter as counterproductive.

“Suddenly, Iran can say to us: ‘Are your proposals actually trustworthy if 47 senators say that no matter what the government agrees to, we can subsequently take it off the table?'” German Foreign Minister Frank-Walter Steinmeier said Thursday during remarks at the Center for Strategic and International Studies in Washington.

“This is no small matter we’re talking about,” Steinmeier said. “This is not just an issue of American domestic politics.”

The letter has shifted attention away from Israeli Prime Minister Benjamin Netanyahu’s speech earlier this month to a joint meeting of Congress. His presence, at the invitation of House Speaker John Boehner, and remarks had forced the White House to defend the framework of a nuclear deal.

Senate Foreign Relations Chairman Bob Corker of Tennessee, who didn’t sign the letter and is shepherding legislation that would require congressional approval for any nuclear deal with Iran, said it wasn’t helpful to his efforts to round up Democratic votes for the measure.

The letter was signed by 47 of the 54 Republican senators, including four who are considering bids for the party’s 2016 presidential nomination.

One of the candidates, Kentucky Senator Rand Paul, said on NBC’s Today Show on Wednesday that the letter was intended to “strengthen the president’s hand” in negotiations.

No Republican has stepped back from the content of the letter, which warned Iran that any agreement they struck with Obama to curb its nuclear program may be reversed by his successor or changed by U.S. lawmakers. McCain told reporters he was “glad” to have signed it.

South Carolina Republican Senator Lindsey Graham said he doesn’t have any second thoughts about sending the letter.

“At the end of the day, I want the ayatollah to understand that our president doesn’t have the ability on his own to waive congressional sanctions without our input and he is claiming he has that right,” Graham said on Fox News. “This is a constitutional crisis in the making. No Senate or House should ever let any president do away with congressional sanctions created by the bodies without their approval.”

Photo: U.S. Senator John McCain speaking at the Arizona Republican Party 2014 election victory party at the Hyatt Regency in Phoenix, Arizona. (Gage Skidmore/Flickr)