MILWAUKEE — Former President Donald Trump chose Sen. JD Vance of Ohio as his running mate on Monday, picking a onetime critic who became a loyal ally and is now the first millennial to join a major-party ticket at a time of deep concern about the advanced age of America’s political leaders.

He says on his Truth Social Network that, “After lengthy deliberation and thought, and considering the tremendous talents of many others, I have decided that the person best suited to assume the position of Vice President of the United States is Senator J.D. Vance of the Great State of Ohio.”

The 39-year-old Vance rose to national fame with the 2016 publication of his memoir, “Hillbilly Elegy.” He was elected to the Senate in 2022 and has become one of the staunchest champions of the former president’s “Make America Great Again” agenda, particularly on trade, foreign policy and immigration.

But he is largely untested in national politics and is joining the Trump ticket at an extraordinary moment. An attempted assassination of Trump at a rally Saturday has shaken the campaign, bringing new attention to the nation’s coarse political rhetoric and reinforcing the importance of those who are one heartbeat away from the presidency.

Vance himself faced criticism in the wake of the shooting for a post on X that suggested President Joe Biden was to blame for the violence.

“The central premise of the Biden campaign is that President Donald Trump is an authoritarian fascist who must be stopped at all costs,” Vance wrote. “That rhetoric led directly to President Trump’s attempted assassination.”

Law enforcement has not yet specified a motivation for the shooting.