Sen. Roy Blunt

In Washington, D.C., Sen. Roy Blunt is on the defense.

The Missouri Republican was one of 47 other lawmakers who signed a controversial letter to the leaders of Iran that warned of Congressional limits in the Obama administration’s authority to unilaterally reach an agreement to roll back the country’s nuclear program.

Speaking Sunday on ABC’s ‘This Week,’ Blunt said the letter was “essentially an op-ed,” and downplayed concerns raised by the White House that it could undermine its negotiations (Vice President Joe Biden called the letter “beneath the dignity of an institution I revere”).

“There is a constitutional role here for the Senate if you want this agreement to be permanent, binding and long-term,” Blunt said in response to questions.

Blunt’s defense was noted Monday by those working to replace him when he runs for election next year. Spokesmen for the Democratic Senatorial Campaign Committee and the Missouri Democratic Party pointed to a stinging editorial in the Kansas City Star on Monday that said the letter “diminished the odds that a peaceful course forward will be found.”

But while Democrats were criticizing Blunt, his likely Democratic opponent – Secretary of State Jason Kander – stayed quiet. A spokesman for Kander, a veteran of the military who spoke out against Blunt’s national security credentials last month, said Sunday that he would not be weighing in on the issue.

Blunt noted that the issue has become something of a “litmus test” for presidential candidates. It was signed by three Republican hopefuls: Marco Rubio, Ted Cruz and Rand Paul.

Former Secretary of State Hillary Clinton, a Democrat who is likely to soon launch her own campaign, said last week she opposed the letter.

“GOP letter to Iranian clerics undermines American leadership. No one considering running for commander-in-chief should be signing on,” she wrote on Twitter.