The Republican governor of South Carolina will endorse Senator Marco Rubio for president three days before the state’s primary, US media report.
Nikki Haley will announce her decision at a rally on Wednesday evening. She said earlier in the week she had not yet made a decision on who to support.
Mr Rubio has been placed third in recent South Carolina polls.
Mrs Haley, one of the state’s most popular Republicans, endorsed Mitt Romney for president in 2012.
According to the State Newspaper, former Florida Governor Jeb Bush was also considered a favourite for Mrs Haley’s endorsement.
Mr Bush recently fell to fifth place out of the six remaining Republican candidates in South Carolina polls.
Mrs Haley gained national prominence after shootings last June at a predominantly black Episcopal church in Charleston, South Carolina, which left nine people dead.
She coaxed the South Carolina legislature into removing the controversial Confederate battle flag – seen as a symbol of slavery to its critics – from the state capitol grounds in the wake of the attack.
Mrs Haley also made some thinly veiled jabs at Republican frontrunner Donald Trump during the Republican rebuttal to President Obama’s annual State of the Union address in January.
“In many parts of society today, whether in popular culture, academia, the media or politics, there’s a tendency to falsely equate noise with results,” she said. “Some people think that you have to be the loudest voice in the room to make a difference. That is just not true.”
Analysis – Anthony Zurcher, BBC North America reporter
Governor Nikki Haley’s endorsement of Marco Rubio could help the Florida senator solidify his hold on third place in the upcoming South Carolina Republican primary. Although that may not sound like a big deal, it most certainly is.
Mr Rubio is currently in a dogfight with his mainstream “establishment” Republican competitors, Jeb Bush and John Kasich, to emerge as the man best positioned to challenge Donald Trump and Texas Senator Ted Cruz as the campaign rolls into a crowded March primary calendar.
Mr Trump’s top position in South Carolina appears secure. He leads by double digits in opinion polls, and surveys show many of his supporters object to Ms Haley’s removal of the Confederate flag from the state capitol grounds. Her endorsement will not sway their votes.
But the Florida senator is within striking distance of Mr Cruz, with whom he has sparred on immigration reform and national security. If Mr Rubio distances himself from his establishment competitors and perhaps overtakes the Texas senator, his political outlook brightens considerably.
Mr Rubio’s campaign was staggered after a less-than-impressive result in the New Hampshire primary. This endorsement gives him a boost when he needs it most.