Donald Trump’s Republican presidential bid has received the backing of Sarah Palin, the populist ex-governor of Alaska who was the Republican vice-presidential candidate in 2008.
Mrs Palin said she was “proud” to endorse the billionaire, in a statement released by his campaign team.
She and running mate John McCain were defeated by Barack Obama in 2008.
Despite retiring from politics in favour of a media career, she remains an influential conservative voice.
Ms Palin is expected to formally announce her endorsement when she joins Mr Trump on stage at rally in Iowa on Tuesday night.
The New York businessman, who leads the Republican race, said her backing was “coveted” and “influential”.
He said she was a “trusted conservative” with a “proven record of being fiscally modest, staunchly pro-life and [she] believes in small government that allows businesses to grow and freedom to prosper”.
- Donald Trump, from mogul to frontrunner
- Sarah Palin, the Tea Party favourite
- 22 things that Donald Trump believes
- US election in depth
Analysis – Anthony Zurcher, BBC News, Washington
While the former Alaska governor’s reputation has been diminished since the heady days of 2008 – thanks to her absence from public office and involvement in series of questionable reality television shows – she is still well liked by conservative Republicans and nearly universally known.
Her backing could help insulate Mr Trump against charges that he’s not a true believer in the cause due to his support for liberal political issues and candidates in the past.
At the very least, she could add even more crowd-drawing power to a Trump campaign that already fills entire sports arenas.
Will Palin help Trump?
Mrs Palin was just two years into her Alaskan governorship when she was picked by John McCain to be his running mate.
The self-described “hockey mom” soon garnered huge crowds and massive media attention.
After the election in 2009, she resigned as Alaska governor and has since forged a lucrative career as a writer and political commentator.
Ahead of the announcement, Mrs Palin tweeted a link to an article by her daughter Bristol attacking Mr Trump’s main rival for the key Iowa caucus, Ted Cruz.
Iowa is the first chance for voters to have their say in the nomination race.
Mr Cruz has himself praised Mrs Palin saying “without her support, I wouldn’t be in the Senate” – a reference to her backing that helped him to his surprise victory in a 2012 Senate run-off election.
“Regardless of what she does in 2016, I will always be a big fan,” he tweeted.