Stepping into his second high-profile Republican primary in a week, former President Trump strongly backed J.D. Vance, author of "Hillbilly Elegy," in the crowded Republican U.S. Senate primary in Ohio.

Why it matters: There had been a huge behind-the-scenes war over this pick — perhaps the most contested endorsement of the midterms so far, with top GOP figures at odds.

Driving the news: The nod is humiliating for a Vance rival, former state treasurer Josh Mandel, who had gone full MAGA in the race.

• Millions were spent on TV to show primary voters anti-Trump comments Vance made in 2016 and has since renounced.
• Vance dipped in polls but regained his footing, which impressed Trump.
• Trump said in a statement: "Like some others, J.D. Vance may have said some not so great things about me in the past, but he gets it now."

Catch up quick: The winner of the May 3 primary is likely to face Rep. Tim Ryan (D) in November, for the seat being vacated by Sen. Rob Portman (R).

• Last weekend, Trump gambled on Dr. Oz in his race against Dave McCormick for U.S. Senate in Pennsylvania.

Between the lines: Trump thinks Ryan will be a very strong candidate in November, and that Vance is the only Republican likely to win.

• Trump, who puts a ton of stock in debates, was impressed with Vance’s performances — and thought Mandel was embarrassing.
• Trump was soured by clips of a near-physical debate altercation last month between Mandel and another rival, investment banker Mike Gibbons.

Trump watched a recent debate and thought all the GOP hopefuls were terrible except Vance. Trump says Vance "has the look."

• Trump got over Vance's 2016 criticisms of him awhile ago, and has been impressed with his appearances with Tucker Carlson.

The folks involved in this race are some of the most powerful in Republican politics — with Vance drawing support from media (Tucker Carlson), finance (Peter Thiel) and family (Don Jr.).

• Don Jr. and others close to Trump created a positive echo chamber, and pushed back against Republicans trying to portray Vance as a Never Trumper because of his old attacks.

The intrigue: After NBC dropped a story this week saying Trump was likely to endorse Vance, a ton of people bombarded Trump begging him not to — including major GOP and Trump donors backing Mandel.