Presumptive Republican presidential nominee Donald Trump has chosen Indiana Gov. Mike Pence to be his vice-presidential running mate, a pick that could help unify a divided party behind Trump's White House bid.

Trump made the announcement Friday on Twitter, and said he would hold a news conference at 11 a.m. ET on Saturday.

He had planned a news conference for today, but held off in the wake of the attacks in Nice, France.

"I am pleased to announce that I have chosen Governor Mike Pence as my Vice Presidential running mate," Trump wrote in his tweet.

I am pleased to announce that I have chosen Governor Mike Pence as my Vice Presidential running mate. News conference tomorrow at 11:00 A.M. —@realDonaldTrump

Trump and Pence campaigned together in Indiana Tuesday night, then on Wednesday the two spent more time together at the governor's mansion, along with Trump's children who are influential advisers to their father's campaign.

12 years in Washington

Pence, 57, differs from Trump on his proposed Muslim ban and on trade, and is more socially conservative. But party insiders hope he will help unify Republicans left divided by Trump's controversial campaign.

Pence has the legislative experience Trump is looking for, having spent 12 years in Washington in the House of Representatives.

In Congress, he was not shy about clashing with Republican leadership. He brings a certain anti-establishment attitude, which Trump has campaigned on, yet he still knows how to work with people on Capitol Hill.

Pence calls himself an advocate for limited government, fiscal discipline, national defence and traditional moral values. He is a committed Christian and putting him on the ticket would appeal to that bloc of voters, some of whom question Trump's commitment to social conservative values.

Christian and conservative

"I'm a Christian, a conservative, and a Republican, in that order," Pence is fond of saying.

At the same time, Pence's social conservative credentials could alienate some independent voters whom Trump is trying to woo.

Pence is also an attractive choice for Trump because of his Indiana roots, which could help the New York businessman broaden his appeal nationally.

Pence is a poster boy for small-town, rural America and his polite, Midwestern personality could balance Trump's larger-than-life one that turns off some voters. Pence would not give Trump any competition for the media spotlight.

Pence, who had been seeking a second term, had until noon Friday to withdraw from the Indiana governor's race.

He withdrew Friday morning, as state law bars him from running for governor and also appearing on the ballot as a candidate for vice-president.

A 'divisive choice' say Democrats

The campaign chair for Democratic presidential candidate Hillary Clinton criticized Pence as a "divisive" choice.

"By picking Mike Pence as his running mate, Donald Trump has doubled down on some of his most disturbing beliefs by choosing an incredibly divisive and unpopular running mate known for supporting discriminatory politics and failed economic policies that favour millionaires and corporations over working families," said John Podesta.

Among Pence's initiatives in Indiana was signing a law that critics said would allow businesses to deny service to gay people for religious reasons.

Trump is to be formally nominated as the presidential candidate for the Republican party at a convention in Cleveland, Ohio, on Monday.

Top party officials are already in Cleveland, where a committee voted late Thursday to rebuff a push to let delegates vote for any presidential candidate they'd like. It was a major blow to Republican foes of Trump who have been working to try to thwart his nomination.

The convention could draw 50,000 people and has been classified as a special national security event by the federal government and will have tight security, according to a Republican spokesman.

The Democratic convention the following week in Philadelphia will have similar security, both conventions patrolled by more than 3,000 personnel from Department of Homeland Security.