Donald Trump won formal nomination on Monday as the Republican presidential candidate and picked a right-wing loyalist for running mate, kicking off a triumphalist party convention in the wake of last weekend's failed assassination attempt.

Trump announced 39-year-old Ohio Senator J.D. Vance as his vice presidential pick, rewarding a one-time harsh critic who became one of his most reliable - and uncompromising - supporters in Congress.

Trump, 78, is guaranteed a hero's welcome at the convention in Milwaukee, where delegates delivered their formal nomination two days after the scandal-plagued former president survived an assassination attempt at a rally.

"As Vice President, J.D. will continue to fight for our Constitution, stand with our Troops, and will do everything he can to help me MAKE AMERICA GREAT AGAIN," Trump posted on Truth Social.

Vance had been widely expected as Trump's pick. He will bolster the ticket on the right wing, but with less chance of expanding appeal to more moderate voters and women.

The standard-bearer for a new kind of populism that has come to the fore under Trump, Vance is also one of the least experienced VP picks in modern history.

But he embraces the ex-president's isolationist, anti-immigration America First movement. Vance is further to the right than his new boss on many issues including abortion, where he embraces calls for federal legislation. (AFP)