Republican vice presidential nominee J.D. Vance gestures at the Fiserv Forum in Milwaukee, Wisconsin, US. Reuters

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.

"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," Trump said in a post on his Truth Social network as the Republican National Convention got underway in Milwaukee.

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.

Donald Trump joins Republican vice presidential nominee J.D. Vance at the Fiserv Forum in Milwaukee. Reuters

"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.

Vance, Trump said, "will be strongly focused on the people he fought so brilliantly for, the American Workers and Farmers in Pennsylvania, Michigan, Wisconsin, Ohio, Minnesota, and far beyond.” Several of those Midwestern states are expected to play a critical role in November’s election.

Trump and Vance spoke about 20 minutes before the Truth Social post and Trump formally offered him the job, according to a person familiar with the call who, like others, requested anonymity to share the private conversation.

Florida Sen. Marco Rubio and North Dakota Gov. Doug Burgum, the other contenders on Trump’s shortlist, had been informed earlier on Monday afternoon that they were not his pick, according to people familiar with their conversations.

Associated Press