Dominic Tierney writes: Working its way through the congressional digestive tract like a poison pill is one of the worst ideas in modern legislative history: a bill that would make it illegal to conduct diplomacy with Iran.

In an almost unprecedented move, the Iran Threat Reduction Act of 2011 (H.R. 1905) includes a clause that reads, “No person employed with the United States Government may contact in an official or unofficial capacity any person that … is an agent, instrumentality, or official of, is affiliated with, or is serving as a representative of the Government of Iran.”

The notion of outlawing contact with Iran is one of those ideas that at first glance sounds merely awful — and then upon reflection, seems truly dreadful.

The United States does not have formal relations with Iran but Washington engages in a variety of unofficial contacts, most of which would become illegal. The bill would outlaw discussion with Iran about ways to end its nuclear program, even though this is a supposed aim of U.S. foreign policy. It would also stop the United States and Iran from cooperating in areas like Afghanistan, where there is actually some overlap of interests in avoiding a Taliban resurgence.