Republicans face heavy midterm losses unless an Iran deal lands first, which would be bad news for the party in power and a swing factor for markets.

• The party holding the White House has lost control of the House in almost every midterm since World War II, with only two exceptions.
• Without an Iran deal, a Democratic landslide looks likely.
• A deal would give Republicans a real shot at keeping the House, making it a matter of party survival for Trump.
• That political pressure is itself a reason to expect a deal, which points to an optimistic read on the coming months.

Outlook: Expect a hard push for an Iran agreement before voters go to the polls, since Republican control of Congress may depend on it.