Of all the criticism out there aimed at the Gang of 47 who signed The Letter to Iran, urging the mullahs to reject a nuclear deal with the United States, one point rings truer than the rest.

It is that The Letter has given ammunition to the hardline faction within the Iranian government to use against whatever the so-called “moderates” bring to any discussion on this matter.

Who would have thought that the hardline Iranian Islamic fundamentalists would find allies within the Republican majority that controls the United States Senate?

Roll that one around for a bit.

Freshman Sen. Tom Cotton, R-Ark., drafted the letter and sent it to his GOP colleagues. Forty-six of them signed it, with seven Republicans declining to put their names on The Letter.

They’ve interfered directly with a sensitive U.S. negotiation with Iran over how to persuade the rogue nation to discontinue its nuclear development program. The Gang of 47 well might have broken U.S. law prohibiting such unauthorized negotiation with a foreign power, but the gang won’t be punished for it.

Conservatives think they’re doing the right thing. Liberals are angry with them for undermining the president of the United States, the secretary of state, and our allies who’ve joined us in seeking an end to the Iranian nuclear program.

And, yes, they’ve given the Iranian hardliners reason to smile today as they look toward the United States and see that members of our “loyal opposition” are proving to be not quite so loyal. They’ve turned a bipartisan U.S. foreign policy endeavor into a partisan contest.

The late, great Republican U.S. Sen. Arthur Vandenberg of Michigan, who coined the phrase that partisanship “ends at the water’s edge,” is spinning in his grave.