The widow of murdered right-wing activist Charlie Kirk has endorsed US Vice President JD Vance for president in 2028, firing an early starting gun on the White House race, and offering the backing of the influential youth organisation founded by her husband.

Erika Kirk, whose husband's Turning Point USA was a major player in mobilising young people to vote for Donald Trump in 2024, told thousands of attendees she was backing the vice president to become the 48th president.

"We are going to get my husband's friend JD Vance elected for 48 in the most resounding way possible," she said on Thursday night at AmericaFest, the first major Turning Point gathering since Charlie Kirk was killed.

Vance is due to speak at the gathering on Sunday.

The endorsement comes as the Make America Great Again (MAGA) movement begins to look to a future without Trump.

Vance has not yet committed to running in 2028, but he is widely expected to put himself forward.

Vance was close to Charlie Kirk in the months and years before he was shot dead on a Utah college campus, in a political assassination that shocked America and sent conservatives into shocked mourning. (AFP)