If you’re haggling with a car salesman, it’s a bad idea to punch him in the face in the middle of your negotiations. He will probably be irked, and it will be pretty hard to persuade him not to call security, let alone budge on the price of that 2008 Honda Civic.

The same concept applies to international politics. When U.S. diplomats are trying to hash out a deal with a foreign country such as Iran, it’s not so great when everyone’s favorite train wreck of a political institution, the Senate, gets the urge to impose sanctions on the very nation we’re trying to deal with. But that’s exactly what’s happening.

The interim deal between Iran and the United States, France, Russia, the United Kingdom, China, Germany, and the European Union went into effect on Monday and will last for six months. It lifted $7 billion worth of sanctions on the oft-vilified power in exchange for giving more access to nuclear facilities to international inspectors and halting further development of the nuclear program.

Not bad. The only problem is that the Senate has been threatening to impose new sanctions on Iran, which would be a bizarre slap in the face after America’s diplomatic efforts. The Senate bill to go back on our word and punish Iran for being foolish enough to negotiate with the United States currently has 58 cosponsors (43 Republicans and 15 Democrats, including Sen. Chuck Grassley, R-Iowa). A number of senators haven’t stated their position, so it’s hard to say for certain if there would be enough votes to override a veto from President Obama.

This is scary. There’s virtually never that level of bipartisan support for anything, and now that there is, it’s for something that would effectively sabotage diplomatic efforts to ease the hostile relationship between Iran and the West. It’s not hard to see additional sanctions completely ending this round of diplomatic talks.

Jeffrey Goldberg took it a step further in a recent column for Bloomberg, “An Iran Hawk’s Case Against New Iran Sanctions.”

“If these negotiations were to collapse — and collapsing the negotiations is the goal of some of the most hawkish hawks — the most plausible alternative left to stop Iran would be a preventative military strike, either by the U.S. or by Israel (Arab states, which are agitating for an American strike, wouldn’t dare take on the risk of attacking Iran themselves).”

Does the Middle East really need any more instability with the Syrian civil war spilling into neighboring countries, Iraq’s renewed trouble with militants, or India’s and Pakistan’s traditional squabbling, with occasional violent episodes?

Furthermore, do we want to risk wasting more lives and money fighting another pointless war in another Asian country? At this rate, we might as well just invade the whole region.

Some of this strange behavior from U.S. senators is probably simple political opportunism. Most voters don’t like Iran, so the political risk is minimal. Midterm elections are coming up, so looking tough is politically safe. Israel also doesn’t like Iran, so there are political points to score with that ally.

The benefits of imposing more sanctions on Iran are all in the short run, however. The long-run consequences are potentially ruinous for everyone involved. Let’s avoid socking our adversary in the jaw in the middle of negotiations.