Big day of primaries points to a nail-biter in the fall

After the biggest primary day of the US midterm election season, both parties have reason to be hopeful. But barring some game-changing development, control of Congress is shaping up to be a cliffhanger.

Heading into Tuesday’s eight primaries, including in all-important California, momentum appeared to be shifting slightly in favor of Republicans, bolstered by a strong economy and President Trump’s messaging.

Those influences were clearly a force at the ballot box. In California, for instance, Republican businessman John Cox advanced to face liberal frontrunner Lt. Gov. Gavin Newsom in the race for governor in the fall. That was not a given in this heavily Democratic state, which promotes the top two primary vote-getters, regardless of party.

The president’s endorsement of Mr. Cox,