Republican presidential candidate Donald Trump, right, appears with Indiana Gov. Mike Pence at a campaign stop at the Grand Park Events Center in Westfield, Ind., on July 12. (John Sommers II/Reuters)

In a stunning move late Thursday, Donald Trump said he was scrapping his plans to announce a running mate because of the terrorist attack in southern France, following a day of strong signals that Indiana Gov. Mike Pence was the likely choice.

Throughout the day, aides to Trump, the presumptive Republican presidential nominee, were preparing to formally announce Pence as the vice-presidential candidate at a news conference in New York on Friday morning. But by early evening, Trump said that he had yet to make a “final, final decision” between Pence and two other candidates, New Jersey Gov. Chris Christie and former House speaker Newt Gingrich (Ga.).

With the Republican National Convention just days away, it was unclear when Trump would finalize or announce his selection. But he won an important victory as Republicans setting the rules of next week’s convention squashed attempts to rob him of the GOP nomination.

Trump cited the attack in Nice as his reason for postponing his vice-presidential announcement, although that was the only part of his campaign he was suspending. On Thursday night, he continued with fundraising events in California, and he called in to two Fox News Channel broadcasts for interviews.

Indiana Governor Mike Pence (R) is on Republican presidential candidate Donald Trump's short list of potential vice presidential candidates. Here's what you need to know about him. (Peter Stevenson,Danielle Kunitz,Osman Malik/The Washington Post)

Pence was spotted Thursday evening in New York, an indication that he had been chosen, while Gingrich had no plans to be in the city, according to several Republicans familiar with the process.

Trump sparked intense speculation as he held off throughout the day on notifying Pence or any other potential picks of his decision, said these Republicans, who spoke on the condition of anonymity because of the confidentiality of the ongoing talks.

Campaign officials cautioned that the selection process remains fluid — and although they did not dispute that Pence probably would be chosen, they noted that Trump still could spring a surprise.

For Pence, the timing is crucial: Under Indiana law, he has only until noon on Friday to have his name removed from the November ballot for his reelection bid. That step would be required if he were the vice-presidential nominee.

In an interview early Thursday evening with Fox News Channel, Trump said, “I haven’t made my final, final decision. I mean, I’ve got three people that are fantastic.” He went on to praise all of them.

Trump had been torn between following his gut instincts to tap a fiery combatant such as Gingrich or Christie and heeding his advisers by selecting the soft-spoken and low-profile Pence.

Pence’s elevation to the ticket could help unify the divided Republican Party ahead of next week’s national convention in Cleveland. Early reports that Pence would be chosen were welcomed on Capitol Hill, with House Speaker Paul D. Ryan (Wis.) calling the governor “a good movement conservative.”

A deeply conservative former congressman and talk-radio host, Pence, 57, is a seasoned politician who could help bring together disparate blocs of the Republican coalition. Trump would rely on Pence especially to bring aboard social conservatives and establishment leaders who remain skeptical of, if not outright hostile to, Trump’s candidacy.

Trump has long said he wanted a running mate with governing experience who could help him enact his agenda in Washington, and Pence’s credentials as a former House Republican leader seem to fit the bill.

However, Pence’s gubernatorial tenure has been marked by controversy over a state law considered discriminatory against gays and has alienated Democrats, who consider him a rigid, socially conservative ideologue.

Pence has not always agreed with Trump’s policy ideas. In December, for instance, the governor criticized Trump’s controversial proposal to temporarily ban Muslims from entering the United States. “Calls to ban Muslims from entering the U.S. are offensive and unconstitutional,” he tweeted.

On trade, Pence and Trump have been on opposite sides. While Trump campaigns as a strident protectionist, opposing the Trans-Pacific Partnership and vowing to renegotiate the North American Free Trade Agreement, Pence has been a proponent of such deals. As a member of Congress, Pence voted for every free-trade agreement that he faced.

Throughout Trump’s weeks-long deliberations over a running mate, his political advisers, including campaign chairman Paul Manafort, have urged him to select Pence, people familiar with the discussions said.

But the candidate’s adult children, as well as his son-in-law, Jared Kushner, have given him differing advice at times. Donald Trump Jr., the candidate’s eldest son, and Kushner have advocated for Gingrich, people close to the family said.

Trump Jr. insisted in a note Thursday that there has been “no rift or lobbying whatsoever” and described the family’s conversations about a running mate as “very calm and organized.” He said that he admires Gingrich and Christie, and that he hasn’t known “Mike as long but was very impressed” when they met on Tuesday and Wednesday.

“It’s about who will mesh best with my father,” Trump Jr. wrote.

All week, Trump has built suspense around his deliberations. He held campaign events with the three finalists to test chemistry and to measure the reaction of crowds. He held a series of meetings in Indiana on Wednesday. And on Thursday, even as word leaked that Pence was the likely pick, Trump played coy.

Republican lawmakers responded to Thursday’s reports with warm words for Pence, but cautioned that they, too, had received no word on the selection from Trump or his campaign.

“I’m happy for him and happy for the ticket,” said Sen. Bob Corker (Tenn.), who advises Trump on foreign policy and other issues.

Although Corker added, “I don’t know for real that this is it. . . . I’m surprised. I would have thought they’d be waiting until tomorrow — usually people wait until the last minute to make a decision. So that’s why I’m a little cautious.”

Trump is known to value loyalty and those who have supported him from the early days of his unconventional campaign. Pence endorsed Trump’s chief primary rival, Sen. Ted Cruz (Tex.), shortly before the Indiana primary in May. Although the endorsement was notably tepid and seemed designed to offend Trump as little as possible, they were clearly at odds with each other.

With his comfortable victory in Indiana a few days after that endorsement, Trump effectively clinched the nomination — knocking out his final two opponents, Cruz and Ohio Gov. John Kasich. The sweeping Indiana victory remains a point of pride for Trump that he often celebrates on the campaign trail.

Sen. Jeff Sessions (Ala.), considered a long-shot vice-presidential prospect but one who has grown close to Trump, also visited with Trump in Indiana and traveled with him to California late Wednesday. Retired Lt. Gen. Michael T. Flynn, a registered Democrat, was another person Trump considered in recent days.

But all morning Thursday, there were signs that Pence had become Trump’s top choice. The governor convened an 8 a.m. meeting in Indianapolis with political allies to go over logistics should he join the ticket, Time magazine reported.

Also Thursday, Marc Lotter, Pence’s deputy campaign manager, was spotted by reporters on a flight from Indianapolis to New York, where Friday’s announcement would have been made.

The moves had Pence associates in Indiana abuzz about his likely selection. One of the governor’s top advisers and fundraisers said, “Most everybody in Indiana thinks it’s Pence.”

The hope within Trump’s orbit has been that he could win plaudits from powerful Republicans for choosing someone they largely find acceptable — and that he could get a fresh look from general-election voters who have been eager for signs of seriousness from the combative businessman.

Indeed, many GOP lawmakers praised Pence on Thursday.

“Mike Pence has the legislative experience, having been in Congress and been a leader here,” said Sen. John Barrasso (Wyo.). “I think he’d be a good choice, but that doesn’t mean the other guys aren’t good choices, too.”

Rep. Luke Messer (Ind.) said picking Pence would be a wise choice by Trump.

“He makes the ticket better from day one,” Messner said. “I think it will be an important step toward bringing the entire coalition behind the Trump ticket.”

But some more moderate Republican lawmakers said they were concerned Pence is too rigidly conservative to help Trump with the general electorate.

“If the objective is to broaden the appeal of the party beyond the base, I’m not sure this would be the wisest choice,” said Rep. Charlie Dent (Pa.), noting Pence’s positions on social issues. 

Meanwhile late Thursday, Republicans responsible for setting the party’s presidential nomination rules overwhelmingly rejected attempts to formally unbind delegates to the convention, effectively killing serious attempt to rob Trump of the GOP nod.

A vote came quickly Thursday night as part of a day-long marathon session of the Republican National Convention’s rules committee, which sets the rules of the meetings and how the party will pick its nominee in 2020.

The rejection of the “unbinding” proposal was so overwhelming that committee leaders opted not to record the tally. Trump supporters on the committee quickly exercised procedural tactics to effectively kill any attempt to revive the issue — and to codify that delegates are indeed bound to vote for the results of state caucuses and primaries.

Manafort quickly heralded the win via Twitter: “Anti-Trump people get crushed at Rules Committee. It was never in doubt: Convention will honor will of people & nominate @realdonaldtrump.”

Dan Balz, Karen Tumulty and Ed O’Keefe in Cleveland, and Sean Sullivan, Kelsey Snell, Karoun Demirjian, Amber Phillips and Jim Tankersley in Washington, contributed to this report.