Businessman Dave McCormick officially conceded to Mehmet Oz in Pennsylvania's Republican primary, making Oz the official GOP nominee to run for the state's open U.S. Senate seat.

Driving the news: The extremely tight race between McCormick and Oz had gone into a recount but McCormick was still falling short. The Trump-backed Oz will face Pennsylvania's Lt. Gov. John Fetterman, who is recovering from a stroke, in November.

The big picture: A recount of the results in the Senate Republican primary was underway as McCormick conceded.

• The race was largely seen as a test for Trump's grasp on the Republican Party. Trump endorsed Oz, a celebrity TV doctor, early in the primary and Oz had declared himself the presumptive nominee as the recount went on.
• Trump supporters in Pennsylvania booed the mention of Oz's name days before the primary.
• It was also one of the most expensive U.S. Senate races in the country with millions of dollars contributed from across the nation, Axios’ Mike D'Onofrio and Taylor Allen report.
• Oz put at least $12 million of his own money into the race. McCormick spent at least $11 million.

The GOP primary battle became a fight over mail-in voting too. McCormick filed a lawsuit in a Pennsylvania court at the end of May to make sure all Republican mail-in ballots submitted without a handwritten signature could be counted.

Recently, Democrats haven't been waiting around for a recount vote having already organized a campaign blitz against both candidates, Axios' Alayna Treene reports.

• The race to replace retiring U.S. Sen. Pat Toomey (R), will be nationally watched and the outcome could determine control of the upper chamber.