Sen. Tom Cotton and the White House are engaged in a testy back-and-forth over the issue of buying heavy water from Iran.

“The White House may think it’s a laughing matter to subsidize Iran’s nuclear program, but I don’t,” the Arkansas Republican said on MSNBC’s “Morning Joe” Thursday morning. “Heavy water is not radioactive, not dangerous in normal quantities, but it is an essential part of any nuclear program like Iran’s.”