Republican Donald Trump has selected Indiana governor Mike Pence to be his vice-presidential running mate, according to US media reports.
Mr Trump's campaign planned to announce his selection on Friday, but cancelled the event because of the attack in Nice, France.
Sources close to campaign told ABC News that Mr Pence accepted the offer.
However, Mr Trump told Fox News on Thursday evening: "I haven't made my final, final decision."
Mr Trump hopes Mr Pence can help him appeal to the party's conservatives.
Other candidates reportedly in the running were former House Speaker Newt Gingrich and New Jersey Governor Chris Christie.
Analysis: Anthony Zurcher, BBC News, North America Reporter
If there's anything resembling a "safe" vice-presidential pick for Donald Trump, Mike Pence is it. He has executive experience as Indiana's governor and a strong legislative resume from his 12 years as a member of the US House of Representatives.
While in Washington, he chaired the Republican Study Group, a coalition of hard-core conservatives, which gives him solid bona fides among the grass-roots Tea Party wing of the party that has occasional doubts about Mr Trump's ideological purity.
Mr Pence also hails from the mid-west, which Mr Trump's team has identified as perhaps the key battleground in his quest for the White House.
In Republican circles Mr Pence's record isn't entirely clean, however. Some on the right have criticised the governor for backing down when the state's "religious liberty" law was challenged by LGBT activists and local businesses last year.
Mr Pence's decision to expand government health-care coverage for Indiana's poor is also considered ideological heresy by some.
The real question, however, is whether Mr Pence has the rhetorical dexterity to both fulfil the traditional running-mate role of political attack dog on the stump and the nominee's most ardent defender.
Before his current job, Mr Pence, 57, spent 12 years in Washington as a congressman.
His legislative experience and position of governor of a Midwestern state could give Mr Trump advantages in the general election.
The Indiana governor is strongly anti-abortion and signed a religious freedom bill, which some saw as anti-gay, into law.
The campaign hopes that Mr Pence will help boost Mr Trump's image with social conservatives who have been unsettled by Mr Trump's brash persona.
Mr Trump - a New York real estate tycoon who has never held elected office - has said he wants a running mate who could help him work with Congress.
However, Mr Trump and the Indiana governor differ on some key issues including the billionaire businessman's call to ban Muslim from entering the US.
Last year, Mr Pence tweeted that Mr Trump's plan was "offensive and unconstitutional".
He has also expressed support for free trade agreements and was in favour of the war in Iraq, which Mr Trump says he was not.
Mr Pence also criticised Mr Trump for his attacks on Hispanic judge Gonzalo Curiel, who was born in Indiana.
Mr Trump had said Mr Curiel could not possibly rule fairly in a case against him became of his Hispanic heritage.