Donald Trump has abruptly postponed the announcement of his vice presidential running mate because of a deadly truck attack in France, but Republican sources say it was expected to be Indiana Governor Mike Pence.
Pence, 57, has diverging views with Trump on his proposed Muslim ban and trade, and is more socially conservative, but he could help unify a divided party behind Trump's White House bid.
Trump was due to make his official announcement on his choice on Friday at 11am (0100 AEST Saturday) in Manhattan.
But he tweeted on Thursday night that the attack in Nice, where a truck slammed into a crowd killing dozens of people, prompted him to delay.
"In light of the horrible attack in Nice, France, I have postponed tomorrow's news conference," said Trump. He said in a Fox News interview: "We will announce tomorrow when it will be."
Trump, who has proposed banning Muslims from "terror states" from entering the United States, said in another Fox News interview that the attack in France showed the United States and the rest of the world needed to get tougher in the fight against Islamist militants.
"This has to be dealt with very harshly," Trump said.
He told Fox News he had not made a "final, final decision" on a running mate. He heaped praise on Pence and his other two finalists, former House of Representatives Speaker Newt Gingrich and New Jersey Governor Chris Christie.
"I've got three people that are fantastic," he said.
Trump's advisers told national party officials that he had settled on Pence, according to two Republican sources familiar with the campaign's operations.
"I'm told he's been asked to do this and he's flying to New York," one source said. Pence was seen by TV networks arriving at a New York-area airport.
Trump, 70, a New York businessman, is to be formally nominated as the party's candidate for the November 8 presidential election at the Republican National Convention next week in Cleveland. Traditionally, the vice presidential choice is used to build enthusiasm among party loyalists.
Trump's choice of running mate is seen as especially critical because his defeat of 16 rivals in the Republican primary race left the party divided.
Some party leaders are still uneasy about some of his campaign positions and free-wheeling statements, such as his comments on Muslims and immigrants.
"Pence is Donald Trump's straight man," said Republican strategist Ron Bonjean.
"He'll be able to defend him as well as be a cheerleader but do it in a calm, cool, collected manner that will preserve his credibility."
Pence, a former congressman, is seen as a safe choice, not too flashy but popular among conservatives, with Midwestern appeal and the ability to rally more party faithful behind Trump. The businessman has never held elected office.
"He's a good, safe, solid conservative," said Republican strategist Scott Reed.
But Pence is to the right of Trump on social issues, having signed restrictive abortion legislation and pushed to defund the Planned Parenthood women's healthcare organisation, whose services include providing abortions.
Trump has said he opposes abortion, but his views have been inconsistent, and he has said Planned Parenthood provides some valuable services.
Pence has also criticised Trump's proposal to ban Muslims temporarily from entering the country.
