WASHINGTON — His wasn’t the first or largest signature, but the bold, blue strokes used by U.S. Sen. Cory Gardner in a letter to Iran left no doubt that the Colorado lawmaker stood with his Republican colleagues in opposing the White House — and causing a political tempest in the process.

The open letter to Tehran from 47 Senate Republicans that was made public this week has ignited a debate among the chattering class, with Vice President Joe Biden condemning the missive as a “beneath the dignity” of the Senate and The Wall Street Journal labeling it a ” distraction.”

But in an interview Tuesday, the freshman senator defended the decision to add his name to the letter, which emphasized to Iranian leaders that the president could do only so much without the say-so of Congress — even as President Barack Obama is in the middle of what Biden called “sensitive international negotiations” with Iran over its nuclear program.

Gardner said the goal was to make clear that Congress adamantly opposes the possibility of Iran being armed with nuclear weapons and that anything else was a distraction too.

“If you listen to the reaction of the administration, they are in hyperdrive trying to downplay what’s really at stake,” Gardner said. “That’s why the president is trying so hard to distract people from the real issue.”

The Obama administration has emphasized that it also opposes nuclear weapons in Iran and that “all options remain on the table” to prevent that from happening, according to Biden. But its diplomatic dealings with Iran have raised suspicions among Democrats and Republicans on Capitol Hill.

“We’re not just talking about trading goods here,” Gardner said. “We’re talking about the very future of the world.”

Even so, the Republican-only letter has become its own source of partisan controversy. Asked about its long-term effects, Gardner said it does nothing to hinder his campaign goal of trying work the ideological middle of the Senate.

The Iran issue “is a prime example of where we can and should work together,” he said.

The letter, however, represents the second time in Gardner’s nascent Senate career that he has supported Republican leaders in breaching established protocol in the arena of global affairs.

The letter to Iran — spearheaded by another Republican freshman, Sen. Tom Cotton of Arkansas — has been criticized for violating the unsaid rule that U.S. politics should stop at the water’s edge.

In a related situation, Gardner was an ardent supporter of House Speaker John Boehner’s decision to invite Israeli Prime Minister Benjamin Netanyahu to address a joint session of Congress, even though Boehner did not consult the White House in advance.

Gardner not only attended the March 3 speech, he co-sponsored a resolution that welcomed Netanyahu to Washington.

Asked whether he thought that these actions taken together represented a shift in how the U.S. policymakers debate foreign policy, Gardner said he always is willing to work with the White House — as long as Obama wants the same.

“We have a number of challenges ahead of us,” said Gardner, who sits on the Senate Committee on Foreign Relations.

Not every Republican senator signed the letter, however, including Sen. Bob Corker of Tennessee, who chairs that committee.

In a statement, Gardner’s Democratic counterpart, Sen. Michael Bennet of Colorado, applauded GOP lawmakers such as Corker.

“This letter is completely counterproductive and I cannot say enough about the seven Republicans who were wise enough not to sign it,” he said.

Ken Bickers, a political scientist at the University of Colorado, downplayed the significance of the feud and compared the debate over protocol to “arguing over the salad fork.”

He said Congress long has played a vocal role in global policy and that the latest episode is more representative of the partisan divide between Obama and Republican leaders than anything else.

“The protocol questions really are a sign of how fractured things are between Congress and the White House,” Bickers said.

Gardner’s role in the letter, however, wasn’t the only Colorado controversy to come from the Iran debate.

In a tweet posted this week, U.S. Rep. Jared Polis, D-Boulder, took a swipe at Cotton, the letter’s lead author.

“Tehran Tom took his case directly to the Iranian government,” Polis wrote.

The comment led to immediate outrage from the state party, which demanded an apology.

“Unlike Jared Polis, Sen. Cotton is a decorated war veteran who has put the good of this nation before his own personal ambition time and again,” wrote GOP chair Ryan Call in a press release.

Polis, in a statement, said “no one is questioning or minimizing Senator Cotton’s honorable past service” but that he “should know better than most the harm” in undermining the commander-in-chief.